Just finished this a week ago; my custom Deep Sea Spartan.
I really loved the green/steel paint job on the Halo 4 Walgreen's Exclusive C.I.O.. To me it gave the figure a cool aquatic look, so I played up the concept and made a custom deep sea Spartan. I did a helmet swap of the Halo Reach Walgreen's Exclusive JFO and mod it fit a Halo 4 body, then repainted it to match the C.I.O's paint job. Next I figured he would need a D.P.U. (Diver Propulsion Unit), so I made one that fits on his back using fodder from armor ZOID and few other model kits. I painted the top half of the D.P.U. a darker shade of grey/steel than the bottom half, which was painted to match the Spartan's armor. The ideas behind that, was to mimic how large fish usually have a lighter under belly then the rest of their bodies. Once the Spartan and his D.P.U. was complete I began working on the base.
I designed the base to look like a hydrothermal vent, a "white smoker" to be exact. The lower half of the base/vent is made from left over parts from my Glow in the Dark ODST custom; so it's mostly plaster, hot glue, and paste (texture). While the pillar of smoke is made of numerous layers of hot glue parts; that fit together like a jigsaw puzzle. The sponges are made from "greenstuff" and the jelly fish are made from hot glue. I've never seen an aquatic base with jellies, so I figured why not try to make some. The octopus, I call him "Inky", is made of the same material as the lower have of the base/vent. Since octopus can change their color and texture to match their surroundings, Inky is painted the same color as the vent. The lower have of Inky, his sucker disks are painted a dark purple, because I thought red or pink would throw off the camo effect. For fun, I gave Inky a heavily battle damaged Master Chief helmet to play with; it was a left over from another custom project.
The last thing I added to this base was a compartment for a mini blue flash light. Why a blue flash light, because most of the under sea pics of white smoker vents have a blue tint to them. In a dark room the glow effect is bright, but not too bright.
So armed with his trusty Target Locator and with Guilty Spark's assistance through the aybss, this Deep Sea Spartan is ready for an undersea battle.