Please close this bug report, seems that everything works as intended. Ship spells can only be cast on a ship by Aquarians, which confused me.

@robertp: There are two different modifiers for ships. Most spells cannot be cast on outgoing ships. Spells with the ship modifier can be cast on outgoing ships. So the week they are still on land, but sailing out into the ocean. Spells with the Sea Spells modifier can also be cast by non-auarian people on the ocean. This is probably a bit confusing at the beginning. Sea people can cast all spells on the ocean, regardless of the modifier.

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