Welcome to the forum.
I've found Realistic Water for as cheap as $12.50 online...you just have to search around a bit for the supplier that's dumping it. Woodland Scenics also makes a product called E-Z Water. It comes as a 16oz bag of crystals that you need to heat to dissolve and then pour. It runs between $7.50 - $9.00 depending on where you get it.
As far as commercial products go, there are several options. I have heard of modeler's using clear resin, like that used on bar tops, but if anything, it's usually more expensive than the modelling products unless you buy it in industrial size containers. If you don't mind an opaque look, I've also seen dioramas that have semi-clear silicone for water but this will not self-level, so that's a drawback. Some of the best water I've seen on a ship diorama was actually made from plaster and painted to look like the ocean.
It really comes down to the specific application and the look you want.