You need to consider where the water will go IF you are successful. Does your deck have a distinct slope AWAY from your house. You need about two inches of slope or more. 1/8 to 1/4" per foot Many decks are level. They shouldn't be, they should "drool" away from the house.
As far as caulking the seems, I doubt you can be 100% successful. Best shot would be to thoroughly clean the crack and use "swimming pool caulk". Something by Thiokol or Pecora that is soft and stretchy.
Another approach might be to create a series or troughs between the joists to catch the drool and carry it away. I built one like this about 35 years ago, but we installed the troughs before we put in the decking and the decking was like a series of pallets that could be lifted out occasionally to clean up. As far as I know, its still working.