Thats like asking someone to buy you a car with no input... lol
What do you plan on doing with it?
Do you plan to do a lot of cruising? Do you wanna get where you're going as fast as possible? Do you plan to wakeboard/tube/ski a lot?
A great motor to cover all of those bases would be a Mercruiser 4.3 (based on the GM 4.3). If you want to do tons of cruising, then the 3.0Merc is a great choice, they are pretty bulletproof but gonna lack a little in the pulling a couple dunkin' lovin' middle age chunkers on slolom skis...
Personally, i think the 5.0 is pointless, they don't have much more power than a vortec 4.3 but just worse gas mileage...
The 5.7 is also a great choice, but not gonna be pretty on the wallet if you live cruising a lot (this is what my boat has, but I don't spend a lot of time cruiser, pretty much go somewhere and hang for several hours)...
On a side note: my best recommendation would be to find a cheap boat that is falling apart structurally that you can get the donor motor from.. Going to be by far your cheapest option versus buying an entire refurb marine crate motor and having to buy all the extra stuff to go with it.