This is what I have been explained by my dermatologist who is also a continuing student of mine and what he has noticed with this problem.
1. For salt water the salt for some can actually clog the facial pores creating the acne, then add sun to that it creates the redness and to the point of creating a sun burn further blocking the pores that create the sebum. Even though you may not develop a full on sun burn the sun will still cause an effect on some people.
2. If you are continuously applying sunblock to your face this will also block your sebum pores which create acne, some people need to use a different sunblock for the face which is more sensitive than one uses for the body. You need to find a good sun block that works for your face which you would be best to contact a dermatologist as they normally have various types and brands.
3. Moisturizing- this also applies the same as the sun block mentioned above as well as finding a sunblock with a good moisturizer.
4. Before applying sunblock/ moisturizer you need to ensure your face is cleansed very well with a good cleanser and stay away from those with the micro beads for some they tend to irritate the skin more, possibly block pores, and do not break down in the water (these are becoming band in certain areas of the US soon).
5. With cleansing your face you need to have very good clean water, the water used on most boats may not be clean enough (even though its potable) for one with a sensitive face, the water treating process may also have other substances added or missing which can effect ones sensitive face. So you may want to have water just for cleaning your face. This can also be the case for on-land facilities especially in foreign countries with water quality issues.
7. How much mildew is on your mask skirt, is it quite clean?
This could cause irritation to the pores of your skin?
8. Do you use a hood and is it cleaned often?
This could also result in the same as above, even the hood being clean could cause an irritation to the pores resulting in an acne break out.
I would still consult a dermatologist who can evaluate your skin better.