Easiest to replace with same engine and you can upgrade to a vortec 4.3 in some process. Vortec is better head design Chevy came out with in 96. Basically difference is heads and intake. If you have space you might also be able to drop in a 5.7 if your boat was offered with a V8 as well.
New base engines are about $3k. You would need a new intake manifold to upgrade to a vortec 4.3 might be worth getting a 4 bbl carb as well to get an additional 20-30 hp well worth it.
Easy to replace ace if you are decent home mechanic and can get a lifting device to pull the engine. I pulled mine in about 2 hours.spent a week swapping accessories from old to new engine, another two or three hours back in and outdrive on.
I would reccomend michigan motorz motorz as I had a good expereince there, but others offer similar base engines or rebuilds. I was happy I went for a new GM base engine, wasn't that much more than a reman.