Looking at a new boat but it has a 500 in it and want a 500efi. Boat is a 2000 and in great shape. Is it possible or cost efficient to covert to a 500efi.

Best bet would probably be a aftermarket system like Fast EFI. Their easy to install and self learning.

cost efficient...no

The Merc 500 efi motors once they get a few years on them can be a headache in my opinion. Atleast mine was.

Just as much as a 6.0 Ford dually.
Wanna know how I know that?

Boat broke down while out on the water (towed in). Truck broke down on the way home (towed also).

Close.

Leave the crab(s) on them. (And yes I am a EFI guy all the way.)

Research and find a good carb guy that knows the 500HP and have his st your carbs up. They will run and start fine. One fellow boater has the 500HP's, and after bringing his carbs into his guy, his engines sound and run awesome. Chit I think he can start his engines faster then me with my 502 EFI. His is 1 and 1/4 pump, turn the key. He pops them off in like 1/2 engine turn now!

I'm a huge fan of EFI, but the newer carbs these days are pretty good, and the OEM fuel injection seems to get a little touchy after a few years. Carbs are not the worse thing in the world on a boat.

This is a very simple set up and is very user friendly. If you are interested shoot me a PM, I may be able to help.

https://www.holley.com/products/fuel.../parts/550-405