For underwater, there is this sealant i bought from Lowes near the acrylic section i think. Its a silicon adhesive, 100% Silicon. Cones in a blue and white metal squeeze tube. I use it for small patching. Just clean the area, put some on my finger, and spread it where its needed. The only problem with Silicon is that once it hits water, it starts to film over. Best to drain, clean, apply, refill.