Thanks!
My problem is I have to be running my ship at the show this sunday, so have to improvise a bit.
Compared to 20 Ampere DC motor and several servos, I think in the first version I can neglect the effect of the current loss. I have seen people using PC817 with 1.3k resistor, which means less then 5 milliamps extra do not make much weather (I will need to experiment with the resistors a bit). A good mod to consider later however!
I have a 4200mAh battery, so in average runtime would be around an hour. My goal is just make sure I am back to shore when its voltage starts sinking rapidly, meaning end of capacity. Later, I might get an Ammeter added too.
As for ATTiny - I think I even have one, but no programmer for it (I had a selfmade parallel port one, but no parallel port any more on my pc). A few milliamps are not decisive in my case. I have a ton of ProMinis here, so its the easiest way. Wires for the probe and from DC buck soldered directly in the board, and when done, just stuck in a plastic box and epoxied watertight.