Fell in water
Well - Your DS is very likely dead at this point. DO NOT "Just Turn It On to Check" This will almost certainly kill it.
If you have some basic electrical & mechanical skill, you could take it apart and have a look. Here is the general procedure for electronics that get wet:
AS SOON AS POSSIBLE (like immediately) after it gets wet, turn off the device and remove the battery. If it has an internal battery, when you get it open, disconnect the battery immediately.
You will be looking for:
If you have identified any corrosion or burns, best thing would be to chuck it.
If there is no visible damage, then pat dry any wet/moist parts. Let the unit sit open in a warm, dry spot for 24h to fully dry.
Reassemble without turning on.
Charge battery, or put in fresh batteries.
Turn on & hope for the best.
In your case, I would do the above, then with a multimeter - check the battery voltage. Then charge the battery for a few hours and check the battery to see if the voltage has increased. If so, you are getting a charge and the unit should power up if it has no damage.
Hope this helps, but really, it's probably dead. You can also find a local firm who fixes water damaged electronics & cellphones to see if they can help you out.