On normal attacks the turn cost is 4 turns and the danger accumulation is 6. With Hostile Waters active the danger accumulation is 12.
With skirmishes the turn cost is 2 turns and the danger accumulation is still 6 per attack. This means for every 4 turns the danger accumulation is 12 and under Hostile Waters the danger accumulation is 24.
A player who skirmishes on a regular basis accrues twice as many danger points as they would under normal PvP attacks. Due to the escape chance not all of these attacks are successful but they still accrue danger points. Considering the fact that there are already so many protection rules built into the game to ensure that defending players can't be exploited in terms of skirmishes this level of danger accumulation under the Hostile Waters card goes a bit far in penalizing skirmishing players. Furthermore, many nations have skirmish insurance which means that there is little to no loss to the defending player when they are skirmished.
Hostile Waters makes sense when a player has attacked and specifically targets someone else using voodoo. With skirmishing, Hostile Waters serves no purpose as most of the income is generated through the maxed out Piracy tech which generates 70k on each successful attack and not so much from the defenders purse.
For this reason I suggest that the effect of the Hostile Waters card be ignored when skirmish attacks take place. Perhaps then we'll see slightly more PvP activity which is what we want.