May have a chance to get a 1754 aluminum boat super cheap, due to a mechanical issue that the current owner doesn't want to fool with. It's got an older 50 horse Evinrude (similar to the one pictured below). Motor set up for several years before he decided to start using it again. It runs great on a hose in the driveway, revs fine, pumps water well, etc. But, on the lake when it's under a load, it just bogs down and doesn't have any "oomph".

What are the most likely culprits? or is there just not enough info? Also, what kind of reputation do these motors have, in general?

Great motors. Carbs gummed up. Almost guarantee that's the problem. Do a compression check. You can rent the gauge from a parts store. Take out the spark plugs. Screw it in and turn over. Do on both cylinders. As long as they are close, you're good. I think that motor will have around 120. May be around 150.

My 40 Johnson would run great on a hose, but under a load it bogged every time. Ended up changing the powerpack and it ran great.

Jets are clogged up or the fuel hose is clogged/restricted.

Yep my first step would be to replace all fuel line/pump and rebuild the carb,

Anytime a motor sits for an extended period of time you can count on fuel issues.

solid motor

As already said above, rule out the fuel system first. Replace all the fuel lines, completely. New primer bulb too and not a Walmarks kind. Get an OEM primer bulb. Mercury seems to be the most common. Rebuild or buy a new fuel pump. Take the carb apart and clean it. See if you can find a manual to tell you the correct height to set the floats at while you're at it. New gaskets on your carbs too.

Save yourself lots of time and frustration, bite the bullet and get it over with. It won't cost much anyway.

I screwed around with my 115 yamaha for a year and couldn't get it to run right without quickly fouling the plugs. I finally bought rebuild kits for the carbs and haven't changed plugs since. I didn't buy fuel hoses for my motor, I made all of them from 3/8 Gates hose. Next time the carbs come off, I'm putting the OEM fuel lines on it. Too much trouble getting the hoses on. You probably just have one carb on that motor so it will likely be easier.

Even if it turns out to be something else, you're still going to need to replace and rebuild the fuel system. It will not be wasted money.

Try a new spark plug first.

The best advice i can give is to send me the address to the boat before you buy it. So i can go buy it lol. If it has low compression then thats why he is selling it. Gonna cost more than its worth. But might be a simple fix. Take it to a shop if your not comfortable checking it. But compression check is easy. Make sure you know exactly what the compression is suppose to be though before you check it.

And if you check it in front of him...then he might not sell it lol

And a 80s evenrude is one of the best ever made to me. I had a 48hp that never needed a dollar for years. Sold it without touching it and it cranked everytime.

Mine was powerpack, carbs, and collapsed fuel lines.

If I do buy it, it will be because I was able to get it cheap enough that even if I have to buy a motor, it's still a decent "deal". Basically buying the hull and trailer and the motor is just gravy. We'll see if it works out...