We are in a similar boat (pun sort of intended)!
Our youngest started tubing when he was four, though I have friends with kids who started earlier! It's a good first step to getting comfortable being behind the boat!
Skiing or wakeboarding requires a higher level of comfort in the water...I have some friends who started when they were ~5, which is pretty impressive, but I think a few years later is more normal! In addition to having the physical ability, kids need to be comfortable sitting in the water for a bit, and have the patience/attention span to soak up instructions. The other key is finding the right environment for teaching -- can't be a day where there are a ton of boats on the water, need to find a quiet time.
As for watersport toys, not sure this goes in the "sport" bucket, but our kids love the big foam 'lily pad' mat, and where we boat, we see them everywhere. That has helped our kids get comfortable with swimming off the boat, and I get the sense we're not alone in that regard!
