DeepWars by Antimatter Games

So in addition to my Arthurian project, I have taken on another distraction - DeepWars.  I am getting a starter set later this month, and have downloaded the rules from the antimatter website

DeepWars, and its sister game ShadowSea (which, incidentally is on Kickstarter right now: link) are based in an alternate reality of Earth where, in the 17th Century, a fantastic subterran continent is discovered beneath Antarctica, filled with treasures and mystical artifacts.  DeepWars takes place underwater in the subterran oceans.  The game has a nice mix of fantasy, steampunk, and Cthulu type intrigue.  It was their dark mariners that got me interested in the game:

In anticipation of my starter set (I chose the scaly horde), I decided to make a custom gaming mat.  I recently saw a tutorial (link) of making one with canvas, sand, paint, and acrylic caulking.  I wanted to capture the eerie abyssal feel of the game, so I came up with a few test colours:

For a mat, I used a canvas painting dropcloth from home depot.  This one was 4x12 and cut to make a 4x4 mat. It won't quite be 4' long at the end due to the shrinkage, but will be mostly 4x4. Nobody plays on the table edge anyway...

I wanted to do 3 base colours - some purple, deep blue, and green.  I got craft paint from Michaels, and bought a bag of playsand from Home Depot, along with their DAP acrylic caulking that contains a bit of silicone.  I found the playsand was too fine, so in addition to the playsand, I  mixed in some small gravel donated from the side of the road at the end of my driveway.  Here is the setup before I got started:

 Here are some pictures of the wet board:

Here are some pictures of the board today.  It is mostly dry, but there are still some soft spots:

What are those tiny coloured bits?  I bought some small craft shells from Michaels and scattered a few to make it more 'bottom of the ocean-ish'.  Currently, I would agree with your thoughts that the colours are a tad too bright and garish.  My plans from here are to drybrush brown over most of the mat, followed by a light, scattered drybrush of aqua green/blue.  I am also considering doing a wash of dark brown to add some shading, but I am unsure as to how well that will work.  Washes work well on sand bases, but this is more of a paste, so I think it might end up pooling and looking odd.  Any suggestions?