The sky as unless using a ND Grad also then sky will be the first area to blow out ordinarily speaking, generally as a rule I marginally over expose the sky as this makes balancing the exposure fairly easy in RAW processing though never rally more than half a stop over
Well assuming the sky is usually brighter than the water, I would set up exposure as normal with a Grad filter to balance the sky and water so not to blow out the highlights, and set the 10 stopper for the water.
Hi and welcome.
Set it to what you want, try different times. Start with say 2 secs then increase the time, or set the dial to bulb and set the exposure to whatever time you want, the longer the more milky look you get for the sea. Hope that helps.
If this is in normal daylight and hence the 10 stop then you still need to balance the shot for dynamic range so I would usually use the required ND Grad to do this, then set the exposure and then slide in the 10 stop. I'm not very good at calculating the exposure with the 10 stop so take a couple of trial frames to get it right. If it is after sunset or before sunrise then the sky may well not need the grad. Just take a test shot without the 10 stop and check the histogram.
This question seems odd to me. I would think it is no different then any other exposure. Your balancing the light of the entire picture. Are you using the filter on only half of the shoot or all of it?