As a wooden boat owner your problem sounds familiar. For advice and local supply of products, check to see if you have a local wooden boat mechanic. If you are do it yourself type, there are many methods (and many products) for sealing wood planks available at most internet boat supply stores (they also carry fiberglass products). I personally use an adhesive/sealant on large seams with excellent results (no leaks in the boat), you can get these products in tubes for your calk gun and force it into cracks after draining your tub and letting it dry a few days. There are lots of types to choose from, primarialy differing in the ability to take it apart in the future (easy - difficult - perminant). I have used a medium grade sealant (can be removed with difficulty) on a large gap several years ago and have had no further problems. Fiberglas works best to repair small damaged areas as an alternative to replacing the whole plank.