Decks, Patios, Porches and Docks - Standing water under deck

I can see that I have standing water under my deck for a few days after it rains. It appears that water doesn't drain out real well. It doesn't seem feasible to pull up any slats and lay down rocks or sand because most of the screws are rusted over or buried into the wood. I'm worried about mildew/rot as well as a hot bed for mosquitos. Any suggestions?

How large is the deck? How big is the gap between the deck boards? How much clearance under the deck? (Can you crawl under it?) I'm assuming that the ground under the deck is level, or very close to it.

You're right -- you need to get rid of the standing water, for the reasons you mentioned.

If the deck is open on some or all of the sides, you can scatter sand or gravel under it from there. If there are spots that you can't reach from the sides, maybe put some sand ON the deck and sweep it so that it falls between the deck boards.

It took most of my weekend, but I dug out around the deck and got to where I could shovel sand/gravel up under it. I put a yard of sand and over 20 bags of egg rocks. Hopefully the water will runoff into the yard now. thanks for your posts.