Hi Merv
For coastal shots you'd be mainly dealing with straight horizons so as a start a hard edged gradual neutral density should be your priority.
For strengths of ND's I would say (for me) it's evenly split between a 2 (0.6) and 3 (0.9) stop. You will need graduated ND's to hold back the sky in your images.
For full ND's, the popular choice would be a 10 stop filter (3.0, ND1000). This will substantially lengthen shutter speed, in most cases up o 5 minutes. Ideally you'd use the 10 stop together with a graduated ND filter.
For grad ND's I would recommend the Lee system. They are pricey but cheap ones will give a strong unwanted colour cast. For Lee you'd need the Foundation Kit (essentially the filter holder), an adaptor ring to attach filter holder to your lens and of course the filter itself.
For 10 stop, you can get the Lee Big Stopper or the B+W ND110. Using the B+W though (a screw in filter) you'll need align you grad filter first before attaching the complete set up. Most people prefer the Lee Big Stopper because of this but I use this method.