yes, draining is only required when replacing the sand, or repairing broken laterals or winter storage.
As for what to paint it with, I don't know what the thing is made from, but you seem to point to fiberglass.
The gelcoat has worn off, so the best way to fix that is to replace the gelcoat
You can also use tremclad on fiberglass, but it takes MANY coats to bury those exposed fibers, you may be better off getting some marine Gelcoat - it will be a bit more, but probably worth it, painting that over the exposed fiberglass, a couple of coats of that should smooth it again, then just cover it with a paint for fiberglass.
Some automotive suppliers will actually tint your gelcoat so you could do this with only gelcoat tinted the color you want.
My buddy did this with a jetski that had seen better days, it turned out awesome.