Here's some gear I've been mulling over for my Europa-based Eclipse Phase game. Note that these are subject to change, since I haven't quite worked out any balance kinks in them. If you have any suggestions or comments, please share them!
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Webbing (bioware, cyberware) [Low]
Webbing adds thin, retractable membranes between the digits (and sometimes limbs) of a morph, improving swim skill tests by +10. Note that cetacean uplifts and the aquanaut morph do not benefit from this. The aquanaut inherent +10 to swimming could be refluffed as a version of this implant.
Flukes (Bioware, cyberware) [High]
This enhancement has two options. Firstly, a muscular tail may be added to any humanoid morph, with a set of flukes at the end. This improves swim speed from 1/5 to 2/10.
Alternately, the legs of a morph can be replaced completely, changing swim speed to 5/25, but reducing walk speed to 1/5 (the morph will need to flop or drag itself along like a seal).
Streamlining (Bioware) [Expensive]
This alteration modifies the kinematics of motion for a morph, improving one particular movement mode with efficiency in mind. A flying morph may have it's wings and feathers altered, a walking morph may have it's gait and leg structure changed. The result is improved rapid motion, doubling the maximum bonus to movement based on movement skill rolls to +10 (up from +5).
Aquatic Adaptation (Bioware, Cyberware) [Moderate]
This enhancement adds small retractable fins at strategic points on a morph (back, legs, abdomen, etc), similar to webbing, above. In addition, the patterns and texture of skin and hair is modified. This improves a morph's fine motions in an underwater medium, negating half of thr penalty for actions underwater (down to -10).
Existing Implants and Gear
Nanotattoos
Very popular with the aquatic set, as bioluminescence is visible even in deep water, and most Europan life forms "see" primarily with sonar and thermographics.
Lotus Coating (armor mod)
This mod also decreases underwater action penalties by -10, but this effect does not stack with Aquatic Adaptation.
Weapons
Underwater, most ranged weapons are useless or have extremely reduced effectiveness. Exceptions are seeker weapons (which can be designed accordingly), and laser pulsers. Laser pulsers still have reduced range, although fine-tuned Europan variants use different spectra, decreasing defraction in the water (although water density still affects this, the degredation is only noticable at extremely high densities hundreds of kilometers down, with a decrease of 10% range and 1 damage every 500km)