Keep the boat the way it is, sell the V-8 for $500 & buy some more tackle. You'll be money way ahead.
All kidding aside. There is much more to it than just switching engines around as in automotive. You will need a host of relatively expensive parts to convert the engine itself to marine cooling etc.. as well as exhaust parts to adapt to the cobra transom plate, shift linkage, cables, & the misc. list will go on & on. The gear ratio is different than what you have now for your 3.0L engine too.