DivinylsFan Does it help to move the sim closer to the water before telling her to play in the ocean? I haven't done anything with Monte Vista beyond loading the world and looking around, but I've never had an issue with this wish elsewhere, so I don't think it requires child Athletic skill.
If you want to test though, without wasting time on building the skill when it might not matter, you can use MC Cheats (click the sim, MC > Advanced > Skill Level) to set the child Athletic skill to 3. If it doesn't work, or it does work but you want your sim to do the work to earn the progress, set it back to 0 or -1, the latter meaning "has not started building this skill at all."