Looking at buying a 18-20' boat. Most of the time we would have 4-5 passengers & would like to pull a tube. Would the 3.0 be under powered for this? I was thinking the 4.3 would be the minimum & 5.0 or 5.7 preferred.
Regardless of the engine, propping is really important. I can prop a 3.0 litre engine in a 20' to be quite 'lively', and I can prop a 5.7 in an 18' to be a dog. With a 3 litre in a 20', as Scott said, many props. That said, for tow sports, bigger is generally better. But also remember, bigger engine, generally more motion-lotion. And 4.3 in a 20' is fine. (It's what I have, and it's plenty good enough)...
my chaparral 2050 sl (20 and a bit) has a 4.3. we were out with 5 this weekend (mind u 2 were children) and it popped right up and had plenty of guts to have fun at around 3000 rpm. 2800 she just stayed up and threw a nice wake would have been good for boarding, though a little quick (could have trimmed up a bit to fix this). putting the hammer down would have been a bad time for the guy on the tube.
I learned to waterski behind a 30 some hp outboard on an inflatable with wood floor. It was a bit difficult to get out of the water but that just trains you to be a good skier. You only need about 15mph to ski. depending on weight and ski size i guess