There are many ways to get rid of this afterwards. I am sure there are members here more experienced than me who can point you in the right direction.
However, as I noticed from the example: you pointed your camera downwards quite a lot with a wide-angle. (It doen's look like you used shift here to place the horizon higher in frame, right ? ) which is, certainly in these very harsh light conditions (close to midday ?) literally asking for all sorts of trouble with flare, glare and what not.
EDIT: Forgot to mention that the above relates to situation where you have a large part of the frame filled with water which reflects the sunlight coming from above. YMMV if you are shooting architecture midday for example.
So a different sort of advice imho would be: Keep the camera 100% horizontal and crop out sky afterwards or more drastic: don't try to take these long exposures this time of day, unless the sun is shielded by a thick layer of clouds. Using the 10 stop closer to dawn or dusk will even give you 300" exposure times ;-)
my 2 cents, Best Regards, Sander
PS you díd cover the viewfinder right ? that also causes problems, yet those are usually more red-ish.