Hi pistitom
Remember we spoke about this privately in the PM section?i'm sure someone in here will understand this problem,but do you remember me saying to you it may be actually a small bug or something in the game,and not your system?.Just one thing,does it happen on calm days too,as I also get a small fps hit during storms and high seas,when on the conning tower and I look out to a specific compass point (not sure which compass point)
This seems to be a well recognized problem,and i'm sure someone will enlighten you more on this.
Lastly,I am not sure you can alter your viewing range 'in game' but I know it can be done by altering your graphics settings via your desktop,and it normally comes under the name of Anisotropic Filtering.What this does is draws detail further out in the distance in games,and lowering this setting will not draw as much detail further out (appears blurred) other than at closer ranges.I set mine to 8x which gives great detail at distance without too much of a fps hit.
What graphics card do you use,and do you have AF enabled at all?In fact,let me know what all your settings are for the card,I may be able to help you in lowering/tweaking certain ones.