Maybe, here is what you are doing wrong:
You are not going for the truthy/falsy checking in the Javascript which is very important at all times>>>

I don't know right now why we get undefined when we use the find method for <c:BoatReviews> right now - maybe has to do something with renderinf life cycle.And we can't use any methods on undefined object. But here is what I did and it solved the problem:
At BoatDetails.cmp side:
........
<lightning:tab label="Reviews" id="boatreviewtab">
            <c:BoatReviews boat="{!v.boat}" aura:id="refreshBoatReviews"/>
        </lightning:tab>
........
At BoatDetailsController.js side:
......
onRecordUpdated : function(component,event,helper){
        let refreshBoatReviews = component.find("refreshBoatReviews");
       // Here I learned the importance of always checking
       // for truthy and falsy statements. If you donot, then you get undefined or some errors
        if(refreshBoatReviews){
        refreshBoatReviews.refresh();
        }
    },
    
    onBoatReviewAdded : function(component,event,helper){
        //  component.set("v.changeTabFocus","boatreviewtab");
        //  Fifth subunit challenge in Aura components basics
        component.find("changeTabFocus").set("v.selectedTabId", "boatreviewtab");
        
//Were not getting errors on doing truthy/falsy checking error here but better develop the practice than being sorry
        let refreshBoatReviews = component.find("refreshBoatReviews");
        if(refreshBoatReviews){
            refreshBoatReviews.refresh();
        }
    }
........

Now this will work exactly as per the business scenario.;)