How tight together were your boards?
How high off the ground is this deck?
What side of house?

One option may be to use floor-register style vents, like in your house.
Remember....warm/humid air RISES.

Faron

The old PT boards have fused together in most cases, I assume since it was too humid. I live near Trenton and the climate is not that wet generally. Otherwise most have ~1/8th in spacing. The deck is ~30" off the ground on the eastern side of the house.

I wonder if I should try to cover most of the ground underneath with perhaps old vinyl flooring, to minimize water absorption into the ground and later coming back up?

My idea of opening a couple of boards follows the thought of including registers. Has anyone solved a problem like mine in this manner?

Most important is to prevent the ground below the deck from becoming saturated in the first place.

Water draining through the gaps in the decking is the only water that should be entering the space under the deck.

The roof must have gutters, and the rain downspouts must drain well away from the deck. The grade must slope away from the house to prevent runoff from backing up under the deck space. You may need to add some french drains if the grade can not be corrected.

After these conditions are satisfied, then you may proceed with improving the ventilation.