This is a question we get asked a lot!
Softlite EPS Cores are waterproof, however the structure itself can take on water, so its best to patch these issues up when they occur.
The first thing to mention is - Please Be aware of Bodyboard Repair Kits! While bodyboards and soft surfboards may look similar in construction, they often use materials which react differently to these repair kits. Bodyboards (traditionally are generally made of PP (Polypropylene) or PE (Polyethylene), while the softboards are made from EPS (Extruded Polystyrene). These repair kits will destroy the EPS Foam.
The best way to repair your board all depends on where the damage is done. We'll run you through the most common scenarios;
Slick - If you've damaged your slick, your best bet is just to use some solar cured Epoxy Resin. This dries hard, so this is perfect for the slick of your board. When applying the resin, smooth it over with a bit of plastic so it dries smoothly (you can just peel this off after the resin has dried. On a Fibreglass / Epoxy surfboard you could sand it, but this doesn't work so great with the HD slicks of a softboard, so its best to ensure the finish is smooth before the resin sets.
Bubbled Deck - This is a common issue that can occur from over exposure to heat, or even just from age or prolonged use. This damage is generally very "cosmetic" and really shouldn't effect board performance. If it's a bubbled deck, we'd recommend slicing the deck, just enough so you can squeeze some glue in there.
An Epoxy Adhesive Glue works well for this. Just double check packaging,ask at hardware store to confirm it is an EPS safe one, and if you have any doubts, test it on a small sample area first.
Fin chop / Hole - Once again, Epoxy Resin is probably the best option here. If it is in an area, where your hands or feet would go, something that dries a little more supple is suitable.
Marine Grade Adhesive Sealant Sikaflex (available from Bunnings) works a treat.
Another option is Solar Rez UV Cured Wetsuit repair. This dries clear and "rubbery" so there will be no sharp edges. As its Solar Cured, it may require a few layers to ensure the mixture can set.