Are you looking for something that you can apply/brush on to the boards directly to keep them dry, or are you looking for something to 'channel' water away once it falls through the cracks?
My advice to folks is, if it's possible, allow air to get under and around the deck to keep that area dry. Sometimes it means removing a lot of vegetation or lattice or the like. Even vegetation along the house needs to be pruned even though it's not immediatly up against the deck. By allowing air and sun to flow around the house, and under and through the deck, it's kept dry. FWIW, I couldn't be bothered with trying to channel any water away because I have the feeling that some day I may have to do some maintenance on it. It may simple be fixing or whatever but I can't be bothered.
My deck is a good sized one. It's a veranda type of deck. The lowest point is about 16" and the hightest point is about 48". The flowers and shrubbery and such around it is the type that grows low to the ground so air and sun can still get under the deck. Sure, the critters get under there but they don't stay long when they find out that 1) there's no protection from a hose and 2) "we're" noisy neighbors for them! The ground is sloped enough under the deck to allow run-off. That run-off feeds the plants at the border. Erosion isn't a problem.
Give this some thought before you spend $$$ on something that might take more time than it's worth. My best to ya and hope this helps.