Autonomous vessels could also look for high bacterial levels (like red tide, or sewage overflows), that pose a threat to human health. Those of us who use small craft for fun, frequently find out about hazardous bacterial conditions after we have already boated in the area. Perhaps the same is true for fishermen (no gender implication intended). But I'm not sure if fixed (anchored) buoys (which may be less likely to be a hazard to navigation), or flying drones that look for spectrographic signatures, might be more cheaper or more practical.
Trying to spread an enormous net of rescue boats that look for people who
hypothetically might be in trouble over a large area sounds economically impractical, and would in fact pose a hazard to navigation. And for those who carry the transmitters, SARSAT already exists, though I admit that many of us don't carry them, because of the costs. A more practical device for many of us is a VHF marine radio, and flares, which many (but not all) people do carry. For those who take no precautions, it may be too expensive to help in a more effective manner than the US Coast Guard, etc., already does. Also, people who don't carry life jackets or dress for the weather are often impossible to rescue in time, regardless. Unless you have a really good idea I haven't thought of.
Even if you knew, by tracking a radio signal, exactly where to deliver a life raft, it might be a lot faster to deliver it by drone than by autonomous boat.
Now, if you want to look for the Loch Ness Monster...

That's basically the same idea as looking for schools of fish.
I can also imagine a diving platform in water too deep to anchor that uses autonomously maintained station keeping. That sounds so obvious that I bet it's been done, with large boats. It could be done with small craft too.
I wonder if you could likewise keep a pseudo navigational buoy or lighthouse at one location in deep water the same way. Is there any reason to do that?
Again, if you want to pick up a spy or assassin from an agreed upon point, maybe it would be safer to send an autonomous boat than to send a person. I bet they are already used (along with drones) to smuggle drugs, weapons, etc. Though that might bother the person who idealistically said autonomous boats shouldn't be used to kill people. Sorry, but a lot of the people with the time and resources to play with new ideas, have military, intelligence, and smuggling organizations behind them. It's always been that way.
There are already autonomous or RC controlled vehicles that look for and perhaps set off mines. I wonder if autonomous or RC controlled boats exist for the same purpose. Probably.
In cities with canals, autonomous boats could deliver goods much like drones can - but could take more weight.
I haven't given a lot of thought to what autonomous submarines could do - aside from smuggling, which I'm sure is already done a lot. (Though manned subs might be stealthy enough to do the job.) Is there anything they could usefully look for? Unless - hmmm. There are deep water conditions under which submarines are probably very hard to find. Another sub, even a small one, might be able to do it.
How about under water prospecting for mineral deposits?
I don't know, but are underwater black smokers a sign of future volcanic eruptions?
Or - could they look for underwater shocks and internal waves that presage a tidal wave? Though, I'm not sure if an anchored buoy could do the same thing better.