I don’t know indian ocean, but I know this effect from north sea: less fps over water then over land. In my case changing from NVidia driver 457.09 to 465.89 helped. You don’t tell your graphics card - maybe you have AMD?
A little off-topic: I switch off shader cache, too. See Solution for Stutters & Pauses - Bugs & Issues / Performance, Graphics & CTDs - Microsoft Flight Simulator Forums