AddBoatReview.cmp

<aura:component implements="flexipage:availableForAllPageTypes" access="global" >
    <aura:attribute name="boat" type="Boat__c" access="public"/>
    <!-- <aura:attribute name="BoatReview" type="Object" access="private"/> -->
    <aura:attribute name="boatReview" type="BoatReview__c" access="private"/>
    <aura:attribute name="newBoatReview" type="Object" />
    
    <aura:attribute name="recordError" type="String" access="private"/>
    
    <aura:registerEvent name="BoatReviewAdded" type="c:BoatReviewAdded" />
    <aura:handler name="init" value="{!this}" action="{!c.doInit}"/>
    
    <force:recordData aura:id="service" 
                      layoutType="FULL"
                      fields="Id, Name, Comment__c, Boat__c, Rating__c"
                      targetFields="{!v.boatReview}"
                      targetError="{!v.recordError}" 
                      recordUpdated="{! c.onRecordUpdated }" />
    
    
    <!-- Display Lightning Data Service errors -->
    <aura:if isTrue="{!not(empty(v.recordError))}">
        <div class="recordError">
            <ui:message title="Error" severity="error" closable="true">
                {!v.recordError}
            </ui:message>
        </div>
    </aura:if>
    
    <!-- Display the new contact form -->
        <div class="slds-form slds-form_stacked">
            
        <lightning:input aura:id="title" name="title" label="Title"
                         value="{! v.boatReview.Name }" required="true"/>
        
        <lightning:inputRichText aura:id="Comment" value="{! v.boatReview.Comment__c }" disabledCategories="FORMAT_FONT"/>
        
        
        <lightning:button label="Submit" onclick="{!c.onSave }"
                          iconName="utility:save" />
    </div>
</aura:component>

AddBoatReviewController.js

({
    doInit : function(component, event, helper) {
        helper.onInit(component, event, helper);  
    },
    
    onSave : function(component, event, helper) {
        component.find("service").saveRecord($A.getCallback(function (saveResult){
            	
                if (saveResult.state == 'ERROR') 
                {
                    console.log('Error saving review record: ' + component.get('v.recordError'));
                } 
                else {
                    
                    var toastEvent = $A.get('e.force:showToast');  
                    if (toastEvent) {
                        toastEvent.setParams({
                            title: 'Success!',
                            message: 'Your review has been saved successfully'
                        });
                        toastEvent.fire();
                    } else {
                        alert (message);
                    }
                    helper.onInit(component, event, helper);
                    component.getEvent('BoatReviewAdded').fire();
                }
            }));
        
    },
    
    onRecordUpdated: function(component, event, helper){
        
    }
})  

AddBoatReviewHelper.js

({
	onInit : function(component, event, helper) {
        var recordTypeId = null;
        var skipCache = false;
        var boatid = component.get('v.boat.Id');
        
		component.find("service").getNewRecord(
            'BoatReview__c',
            recordTypeId,
            skipCache,
            $A.getCallback(function() {
                var review = component.get('v.boatReview');
                var error = component.get('v.recordError');
                if (error || !review) {
					console.log('Error initalizing review template: ' + error);
				} else {
                     review.Boat__c = component.get('v.boat').Id;
				}
            })
        ); 
        
	},
    
    alertSuccess: function(message) {
		var toastEvent = $A.get('e.force:showToast');  
		if (toastEvent) {
			toastEvent.setParams({
				title: 'Success!',
				message: message
			});
			toastEvent.fire();
		} else {
			alert (message);
		}
    }
}) 

BoatDetails.cmp

<aura:component implements="flexipage:availableForAllPageTypes" access="global">
    <aura:attribute name="boat" type="Boat__c" />
    <aura:attribute name="id" 	type="Id" />
    <aura:attribute access="private" name="selectedTabId" type="String"/>
    
    <aura:handler event="c:BoatSelected" action="{! c.onBoatSelected }"/>
    <aura:handler event="c:BoatReviewAdded" name="BoatReviewAdded" action="{! c.onBoatReviewAdded }" phase="capture"/>
                  
    <force:recordData aura:id="service"
        recordId="{!v.id}"
        layoutType="FULL"
        fields="Id, Name, Description__c, Price__c, Length__c, Contact__r.Name, Contact__r.Email, Contact__r.HomePhone, BoatType__r.Name, Picture__c"
        targetFields="{!v.boat}"
		recordUpdated="{! c.onRecordUpdated }"
    />
    
    <aura:if isTrue="{! v.boat != undefined }">
    <lightning:tabset aura:id="boatTabSet" selectedTabId="{!v.selectedTabId}">
        <lightning:tab label="Details" id="tabDetails">
            <c:BoatDetail boat="{! v.boat }" />
        </lightning:tab>
        <lightning:tab label="Reviews" id="Reviews">
           
        </lightning:tab>
        <lightning:tab label="Add Review" id="tabAddReview">
            <c:AddBoatReview boat="{! v.boat }" />
        </lightning:tab>
    </lightning:tabset>
    </aura:if>
    
</aura:component>

BoatDetailsController.js

({
	onBoatSelected : function(component, event, helper) {
		var boat = event.getParam('boat');
        component.set("v.boat" ,  boat);
        component.set("v.id" , component.get("v.boat.Id")) ;
        component.find('service').reloadRecord();
	},
    
    onRecordUpdated : function(component, event, helper) {
        
    }, 
    
    onBoatReviewAdded : function(component, event, helper) {
        component.set('v.selectedTabId', 'Reviews');
    }
})

onBoatReviewAdded : function(component, event, helper) { component.set('v.selectedTabId', 'boatreviewtab'); }

Hi thanks for the sharing, your onSave(component, event, helper) method in AddBoatReview.cmp helped me successfully initialize BoatReview record. I was stuck in this problem for hours.

However, I'm having a hard time understanding why using component.find("service").saveRecord($A.getCallback(function (saveResult){})) , instead of component.find("service").saveRecord(function (saveResult){}). And this seems like to be why my code didn't work.

Many thanks in advance :)