I know this is a bit of a tangent, but I am a serious tea enthusiast, so I try to stomp out this myth when I see it. While caffeine is technically a mild diuretic, caffeinated beverages will not promote dehydration. The mild diuretic effect is a drop in the bucket compared to the fluids gained from the beverage. The research has been really clear on this. You could drink nothing but caffeinated beverages and be just fine (from a hydration perspective) except under extreme conditions. I did this for 2 months, one summer, just to prove a point and I was VERY hydrated.
But, as to the dry eyes, obviously it is a good idea to get checked out. And, I am not an eye doctor. My medical background is limited to emergency and wilderness medicine. So, frame my answer within that context. There are two causes of dry eyes: low tear production and/or high tear evaporation. The windy environment on a boat lends itself to really rapid tear evaporation. I know, antecdotally for myself, that if I let this go for a while on the boat, my tear ducts can get clogged. (Salty air is likely a contributor, as well.) In fact, I need to make a point of washing my eyes with warm water at the end of the day to avoid problems.
So, my advice is:
1) Get checked out
2) Look for polarized glasses that wrap around for better good protection.
3) Thoroughly rinse your face/eyes with warm water at the end of your sail.
My bet is that this addresses the issue.