I think Playmobil ships are designed for bathtubs at best, mostly because they are dual-purpose in that they have flat bottoms for playing on the living room floor, but happen to ALSO be able to float in undisturbed waters.
That being said, as far as the large tallships, I think they can be made to tread rougher waters with some modification. As I said, simply sealing off the windows in the 3050 will prevent any unwanted water coming in, as can be avoided with the 3286 if you seal off the cannon ports (even temporarily, with tape) when sailing in open waters.
As for the exploration vessels (which I've not yet played with), I gander from the looks of things that they may be a bit top heavy, and may also suffer from taking on water through pane-less windows and openings: again, same solution comes to mind: seal the openings, get rid of any unnecessary weight (including crew if need be, or at least make sure your crew dispersment upon the deck is symmetrical and as low as possible), and if possible, add either a heavier ballast ( self-adhesive lead weights, perhaps) and/or create a custom weighted keel to help stabilize your vessel. You could always fashion some sort of pontoon to your vessel and disguise it as a neighboring dolphin or some other floating accesory; I thought about doing this with the 3286 ship in the way of faux Kraken tenticals reaching up to grab it...... still searching for the perfect cepholopod toy to use; Playmobil's octopus is too small IMO.
But don't fault your playmobil ship too much I think they fall victim to the waves because the waves are not proportional to the scale of the ship; To our tiny playmoships, a minor wave is like a tidal wave by comparison, which would sink a real ship at full scale.
