Lotsawater screensaver Mavericks problem
Check the version you got.
v1.7 works like a charm for me under Mavericks.

v1.7 is available here: http://wakaba.c3.cx/s/lotsablankers/lotsawater

Edit: sorry, it doesn't work exactly as I said. Actually, the water animation works, but the screenshot Lotsawater is supposed to do is a black screen instead.
I assume the v1.7 is not Mavericks (full) compatible.

Edit 2: Developper fixed it!, v1.8 is now available and works like a charm :)

***Edit 3:  1.8 works like a charm on a single monitor, but the second monitor of a dual monitor setup still has a black screen shot. This wasn't the case with version 1.7 on Mountain Lion. I hope the developer has another update in the works.