It seems that starting with the later weapons (sniper, grenade, sticky) increases the chance of you getting that map.
But… When you’re in the middle of some the missions you’ll be locked into a particular map or two until you finish those, meaning you won’t see the seaweed map at all.
I never correlated the seaweed map with higher tier weapons, although I suppose that’s possible. I think it’s more likely that the reason you’re seeing that map when you’re carrying better weapons is because by the time you have higher tier weapons you’ve finished most — or all — of the missions, so the game is no longer keeping you locked in certain maps.
Once all the missions are complete, the maps randomize (although the seaweed one is still the most rare, which is a shame because I find it to be the most beautiful).