Subs

Traveller had an interesting design philosophy.

Once you had powerful fusion reactors and avionics of advanced nature, you
could make bricks fly. So they did. That is to say, if you can
make an M1 fly, and hit things because its main armament is shoot-hit
lightspeed or near that variety, then it is a wonderful plane. Coincidentally,
if armoured enough to take those kinds of hits, it is also a good space
interface shuttle and an okay sub.

Now there were optimized shapes for faster airspeeds, but once you
have Agrav tanks, everything better look like one - no more poorly
armoured fast-movers - they tend to die to easily. No more need for
ground vehicles. All your troops are mobile in flight-capable APCs
with tough armour and an ability to lob nasty missiles and hit things with
pulse lasers of fusion guns.

Their grav sub (in the DGP 101 vehicles) that is a small streamlined grav
vehicle capable of (at a guess) 125 kph with torpedos and maybe a pulse laser.
They had a big ocean floor miner (also for hostile
environments) called the Tiamat - a big multi tracked lumbering brute.

Strikes me that underwater lasers would dissippate quickly (but still work if
they can accurately hit over 6 km in DS2). So reduced range
for them. Same for projectile weapons that are waterproofed -
railguns. Missiles (torps) would work. Sonic weapons might work, or not. (I've
heard two divergent camps...). Nets, clouds of debris, mines, etc. These would
all be good.

My 0.01.