I suppose that we should be grateful that River Otters have a strange compulsion to use boat docks as potties. If it were not so, we would not see them nearly as often as we do.
The otter swam to the boat dock and climbed out. With its tail whipping water drops, it used the dock as a potty.
Although its visit was brief, it did allow a portrait. 
If you’ve gotta go, you’ve gotta go…
Great shots, Alistair! Do you think it might be a bit of territorial marking? Sort of “Hey, there, get off of my cloud” ?
Bill, I have often wondered how much of this is marking and how much is merely toiletry. My previous observations suggest that otters do not return quickly to the soiled territory, so marking may not loom large in their considerations. There were actually two otters involved this time and they climbed onto the dock and pooped in succession (I only showed the first one). My guess is that this was just a bit of matutinal venting.
My guess is this otter was mixing her drinks the evening before…some vespertine binging perhaps.