Hmm yeh. Unfortunately I can't see a way round that. The problem is the game engine refuses to not transiently display ocean tiles that it thought were revealed (by position in it considers a map) on a viewport or map switch. To get round this I had to use TERRAIN_SPACE and tell the game engine that all unrevealed tiles are that (so black even when it display it). Unformatunely it wont recognize this on water tiles so I also had to tell it that unrevealed tiles were land, which is what will be causing the effect you see.
Basically if we want to support either viewports or multiple maps, we need to compromise one way or the other - either you get these pseudo-edges in unexplored ocean, or map or viewport switches can reveal stuff you shouldn't see for a few seconds.
A compromise half-way house would be to use the TERRAIN_SPACE trick only for land, but that would mean outlines of coasts that you should no see will be visible to you on map/vewport switches for the few fade-out seconds.
Obviously I could disable it if multimaps or viewports were not in use, but goingorward I think we'll always have multimaps (and probably iewports too) enabled, so i would essentially be a V250-only partial solution.
The weird thing that is behind this, is that different terrain types on water tiles seem to be getting ignored - as far as you are aware is there anythign special about ocean terrains? Do they need something different in their XML or graphics which might mean the trick would work, but that TERRAIN_SPACE can't be used? (i.e. - if we had a black-tile 'ocean' terrain if those are distinguished in some way)