The unicycle bearings are not designed for under water use and over time I’d expect rust problems in the bearings and outside and on the steel hub shaft. They do sell bearings intended for under water uses but since many of the unicycle bearings are not a standard size it is not likely that they could be found in the correct size. See this link for some underwater rated bearings..
Other then the hub bearings, pedal bearing would also likely not last as long either. Other parts of the frame and wheel would also see some impact, just like leaving the uni out in adverse weather.
Water (fresh or salt) isn’t good for metal but it’s not like your unicycle will fall apart because of it. Your bearings might get rusty, you might get spots on your frame, but it’ll probably be fine. Rusty bearings can usually be dealt with by just riding through it. Or maybe squirting some penetrating oil in the area, or in extreme cases, removing the seals and repacking the bearings, or replacing the bearings entirely. Unicycles with crappy bearings are fine; we do not place extreme demands on our bearings.
I have an amphibious unicycle which had the wheel operated underwater in brackish water for multiple hours. It got a little rusty but it’s fine.
I would do the following:
1.) Spray some wd40 in all your metal to metal joints: seat tube/frame, seat tube clamp, spoke nipples, bearing OD/bearing clamp & bolt.
2.) The wheel radial bearings should be packed with plenty heavy grease that should stay inside, but if you like to tinker remove the retaining rings & open the seals and repack with bearing grease.
3.) The ISIS hub shaft & mating pedal cranks should be packed with grease already, but if not remove and apply.
4.) Also, take a look at your pedals. They spin on a shaft with bearings on both ends. I’m not sure if you should spray wd40 into them, because they will “chase out” grease from the bearings.
I’ve dropped unicycles into rivers and canals a couple of times. Dry it out, ride it. WD40 is a readily available water dispersant as well as an all round cleaning solution and lubricant. Worry less. Unicycles are simple and robust.