great advice... I live in an area of swamps where people / friends swamp/drown their atv's on a weekly basis as we ride the swamps all the time.
Only diff in advice is if gear box took in oil flushing with clean water as your water in the pic's is clearly not clean. We use diesal fuel in the gear boxes, differentials, engines of atv's after they are swamped etc... to get rid of water. 3 - 5 flush fills with diesel all that is needed to get out all the water and it only take a 5 - 10 seconds of running time on a gear box to circulated the diseal enough to help get the water out. No need to run for 5-10 minutes or anything like that. Diesel works great and is cheap compared to gear lube which is too thick or iso which is too thin to suspend solids like sand.
good luck. Easily repairable with a grease gun, some elbow energy, and diesel.