I manged to do the first "real life" test at a nearby pond last weekend.
All in all I can say that manouverability was better than expected! I used the gamepad for proportional control (I don't really think that using the keyboard is more than a backup option). I also have to say that I bypassed the motors "smoothing" code that was introduced with the last Arduino firmware update, because it is not necessary when controlling the ROV with the gamepad (It makes the ROV response very slowly and ruins the advantages of using the gamepad). I found it very easy to hover the ROV with the vertical thruster.
I reduced the throttle value range to -30/+30 (in my scale function in ArduinoPhysics.js) for all 3 motors and found that this quite enough for giving the ROV some speed while making the control of the ROV easier by using the range of the gamepad joysticks better. The lower throttle values will also prolong operting time, so I'll probably stay with this setting.
I had not a single failure or brownout during the whole runtime of 1 hour and 10 minutes (also better than expected). I used 8 NiMH cells with a capacity of 4500 mAh. I measured a maximum overall current of 4,5 A when operating all 3 motors at full speed (under water) with the -30/+30 limit.
For bouyancy compensation, I added a 200 ml air tank at the top side of the ROV (see attached image). It's just a PVC rigid pipe from a hardware store with 2 PVC endcaps, which gives some 200g additional uplift. It's not 100% water tight yet, but I think I'll just epoxy the endcaps to the tube instead of dealing with o-rings. The surplus uplift can easily compensated by adding some extra nuts to the threaded bars at the bottom (front or aft, left or right - however you want to trim the ROV). Or by adding additinonal hardware, sensors, etc., of course.
One disadvantage of the tether coming with the kits is that its not floatiing and therefore can easily get entangled in obstacles at the bottom. I'd like to make it slightly positive bouyant - has anybody ideas for that (like using plastic or wooden pearls with a hole in it every 50cm or so)?