My R31 is a 2019. The water tank access is likely unchanged from earlier years.
You should be able to see the tank via the floor hatch just inside the cabin door -- the same one used to access the water pump, water heater, fuel tank connections, and Webasto fuel pump. You'll have to get your upper body down and in, as if you were going to work on the water pump. The tank is positioned forward of the pump (which is just forward of the hatch) and to the right of the mid-berth. The tank extends from there under the middle of the cabin floor about to the helm. It ends just about under the wine cooler area.
Not sure how much you can tell from hatch vantage point about "algae buildup." You can see the aft end of tank reasonably well, enough to hold a light against it to see how clean the walls in that area look (the tank is semi-opaque white). I think there are other ways to get to rest of the tank that involve disassembling the interior walls of the mid-berth to get to the forward end or portside of it. Those are more complicated and perhaps not for the faint-hearted.
As for a boroscope view, I'm uncertain how much you'd see. Assuming you'd go in through the fill line, just getting to the tank would take a good 4 to 5 feet or more. The tank itself is long (6 ft maybe?). My experience with a nice quality boroscope is that it is hard to manipulate it to explore all areas of a large tank once you feed it down a long narrow access tube. You can get a decent look at the immediate area in and around where the boroscope drops into the tank, but not a lot beyond that.
Hope this helps. Good luck.
Gini
