My Overkill 3E campaign took place on the planet Tau, originally a pleasure world. Tau, like Earth, is covered mainly in water. The climate is subtropical to tropical and is covered in rain forests, jungles and savannahs. Plus thousands of kilometers of coastline, island chains and atolls. Prior to the Great Death there were genemodified dolphins, octopus, seals and of course humans using these nature resources. After the Great Death and it's orbital kinetic bombardment there were a great number more atolls and island chains... The following equipment was used in my Overkill campaign. There were a number of ocean based nations that had use of such equipment. This is "mature" technology. Tried and proven devices that have a low failure rate. Most components are inert (ceramics, plastics, etc.) and effectively foolproof. These devices are based on "ancient" technology, made from plasteel, duralloy and other such ultra-tech materials. Such "ancient" items were available to the PC's but as treasure. The one area where my campaign's technology matched ours or exceeded it was in diving devices. I have always loved GMing water based adventures. I hope that these items can be of some use in your campaigns. 
Note: Prices are in copper pieces. The planet Tau had little in the way of heavy metals, so based their economy on copper/brass/bronze. They excelled at the use of fiberglass, plastics and ceramics however.

SCUBA Gear:
"Pony" reinforced polymer air tank, 15-minute air supply, 150m depth range, 100 cp.

Reinforced polymer air tank, 2 hour air supply, 150m depth range, 500 cp.

Reinforced polymer high compression air tank, 4 hour air supply, 150 depth range, 1200 cp.

Reinforced polymer rebreathing unit, 8 hour air supply, 150m depth range, 2C, 3000 cp.

Reinforced polymer helium tank, 2 hour supply, extends range to 300m, 500 cp.

Wet Suit, 100 cp.

Diving Mask, 23 cp.

Diving Gauge, shows time, depth and air time left, 250 cp.

Swim fins, 15 cp.

Spear gun, 50 cp. (use crossbow stats, reduce range by 75%)

Power Swim Boots, WS 24, 2C each (8 hours), uses small motors to generate water jets; handheld wired control, 1000 cp.

Weighted net, 20 cp.

Tank refill, 10% of original cost

Sealed "Dry" Diving Suit, AC 4 (-20), completely separates the wearer from the diving environment, useful for cold water diving or exposure to possible toxins, rather restricting however, eliminates DX modifiers, includes "Hard" diving helmet, 1500 cp.

"Hard" Diving Helmet, AC 6 (-30), a reinforced polymer helmet, allows wearer to be free of a mouthpiece breathing device and speak normally via short range radio (1C 60 hours, 1000m range), mounted headlight (1C 30 hours, 25m range), 500 cp.

NEWT Suits
A NEWT suit is a hard polymer diving suit. Essentially a man shaped submarine. They maintain a one-atmosphere pressure internally so no decompression time is required. Creating a true "shirtsleeve" diving experience. Special joint seals actually become tighter and more effective at greater depth. AC 7 (-35), rebreathing unit (16 hour air supply), 900m effective depth range, suit is very bulky, eliminates PS and DX modifiers, reducing movement to LS 1 and WS 3, has built-in Power Swim Boots allowing WS 18, has three "fingered" claws that function as Partial Hands (borrowed from Palladiums TMNT RPG), two headlights, left forearm has a built-in mini-spear gun (BD 4, BR 5, 30 round clip, 1C per clip, can be modified to carry drugs or poisons), has short range radio similar to the Hard Diving Helmet, suit requires 4C or 1H power cells to function for 30 hours, suit can mount all five power cells at once to allow emergency back-up power, 10,000 cp. 
Note: This is loosely based on an actual world diving suit.

"SeaHorse" Mini-Sub
This is a true submarine in all respects. Used for research, wreck recovery and of course adventuring. AC 8 (-40), 48 air supply, 5km depth range, HD/EN 30, hp 900, MN 16, Mv 40, A/D 10, 2A (60 hours), pilot and copilot/navigator in forward compartment, separate diving bell has room for four divers, 200kg of internal cargo, two built-in mini-spear guns (BD 4, BR 5, 30 round clip, 1C per clip, can be modified to carry drugs or poisons), two "waldo" style manipulating arms (3m long, three "fingered" claws function as Partial Hands), two torpedo tubes (BD 20, BR 500(6R), Slam), four extra torpedoes are mounted internally in two clips, 100,000 cp.