Very good stuff - strong and a great water seal.  It's what the pros use.  Only thing is, it takes forever to fully cure (5-7 days), so don't rush things if you're using it for an underwater application.  The good news is that 3M makes a fast cure version (in white only) that cures in 24 hours - IF the thickness / depth is 3mm or less.  But even that can take 5-7 days if you lay it on thick.  Small price to pay for a great product.