How non-porous is that tarp and how much of the deck it covers. Any fixture affixed to the deck can be a source of leaks (stanchions, shrouds, handrails, pulpits, mast tabernacle, scuppers, .........) The boat has a fresh water tank, does it have water, could it be leaking? Water can get trapped in the hull stringer below the sole ( it can be gallons) and the boat may show dry until some motion partially releases some of it. Some have suscessfully traced a leak with talcum powder; by laying some talcum powder on the inside walls of the hull when it gets wet it will show where the water may be streaming down. Beware that a leak may travel horizontally before it decides to flow down. Get yourself a manual pump and a bucket to remove water, a wet-vac is not going to do you much good when the boat is in the water away from shore power. With the boat in the trailer there is no indication if there are any leaks in the hull or the thru hulls. Try to find and correct the rain leaks, move the boat with the trailer and keep scooping up water until you feel there is no more water trapped. last insure no rain water is getting through the tarp and that the tarp covers the whole deck. After you get the leak under control put the boat in the water and start checking for any leaks emanating from below.