Sealing seams between outdoor composite decking
Sooner or later the splining will deteriorate, and whatever sealant you put in may as well. Also, you are creating little pools in the gaps that may be breeding grounds for mosquitos in wet weather.

Have you considered a waterproof underlayer instead of sealing the seams?
You could use any of various roof panels, such as pvc, polycarbonate, asphalt composition or even metal.  These would only need to be sealed at their edges, a relatively few linear feet that are easily accessible from underneath. And many of them are relatively cheap. 

This layer can be easily sloped and you could direct drips in whatever direction you choose.