Although we have seen what brushless motors are capable of, why is it not widely used in a boat, i have seen so many build threads where they made boats out of only DC motors... i am new to boats and i have wanted to make one for a long time! Please do tell why you would Pick DC motors for boats as they also heat up and need means of cooling... thank you

Because its cheap, readily available and very easy to control if you go the diy route. It is better to have a reversible ESC for a boat, and brushed reversible ESCs are much cheaper.
But brushed motors are generally more inefficient compared to brushless. Heating is an issue for both brushed and brushless and they both need cooling if you want higher performance.

Simple and non tech reason is BL were not available in the market, where many source are there to get brushed motors like old tap recorder, MIC rc cars. People use to get it from old markets in dam cheap price. Again ESCs are very recent compare to the age or rc boats. Water cooled engines were out of reach and too expensive. Also no other replacement options were available in local market.

So I think Brushed motor got the advantage:)