White residue on Vinyl seat
 

Does anyone know what/why this is happening? It happens after rain or from the morning dew. If there is a few days of no rain or no dew than it disappears completely.

looks like mildew...

Thanks, is there any way to combat this? I use 303 protectant every month, the boat is a center console and has no cockpit cover.

Yup, that’s mold/mildew. While it may seem to disappear after some time in the sun it’s likely still there, just not so visible, and therefore pops back once damp. A good mold/ mildew cleaner like 303 or Star Brite mildew stain remover should be able to take care of it. Spraying with Lysol, waiting a minute, wiping off, and cleaning up with water would also probably work.