Had the same problem with a Valiant 490 before, water coming in through the hull-tube join at the stern port side. Was a lot worse though. Pieces of fibreglass were coming out the drain hole from the under-deck area.
Removed the tube on the port side, to find that the fibreglass was all soggy and delaminating. Ground it all back and built it back up like new. Glued back on tube and she was like new.
Oh ya and tasting the water won't confirm if it's sea or rainwater entering from below deck as seawater tends to taste like freshwater after sitting below deck for a while. Something to do with the fibreglass I think??