I know there is probably a lot on here already if I search but just need some quick input. Im looking at a boat with 502's, very low hours, built by a reputable marine engine builder to basically stock merc specs, carbed 415hp. Wondering about ease of upgrades, nothing too hot, thinking fuel injection for sure and somewhere around 525-550hp down the road, no blower. I see a lot of 525 efi fuel system take offs, would they be an option w/ a few changes, or what else could be done easily?

The 502 is an easily upgraded engine. It is an excellent platform to do pretty much whatever you want. You should be able to easily obtain your hp goals with a few parts. There are a ton of used 500 efi intakes for sale for a reasonable price. They will work fine for the power you want. However, if you think you may want more power in the future, you may want to go with a different intake. The 500 intake runs out of steam shortly after 600 hp. It takes some mods to even make it that far. By the time you buy the intake, new throttle body, extrude hone the intake, shorten the runners, etc., you could have easily bought an aftermarket setup with almost unlimited potential. I like the system from Holley (just the intake and throttle body, no electronics), but Edelbrock also makes one. Like I said, if you think you will stay in the 550 hp range, then the 500 efi intake will be fine.
With the 500 efi intake, bump in compression, camshaft, and a little head work, you should easily make 500 hp. Let me know if I can help.
Eddie