Start with the safest options first. Your D3000 has a built-in sensor cleaning function, so try that first. If the spot remains, use mirror lock-up, remove the lens, hold the camera facing downward, and use a hand rocket blower to blow out loose dust.

Do not use a cotton bud. That’s not recommended for sensor cleaning.

Given that the camera was exposed to moisture and possible dirt, a professional cleaning is the safest choice—especially the first time. One answer also noted concern about possible mold because of the wet tropical exposure, which is another reason to have an authorized repair center inspect and clean it thoroughly.

If you only rarely get sensor dust, professional cleaning is usually the best route. If you frequently change lenses, travel often, or get dust regularly, it can be worth learning proper wet-cleaning methods with dedicated sensor swabs and cleaning fluid. It’s doable, but delicate, and it’s easy to make things worse if you’re inexperienced.

So: built-in cleaning, then blower; if that doesn’t fix it—or because of the moisture history—take it in for professional service.