The Grepolis Provider changed it a few times, the last five years, that's the reason, why there are a few different answers for this.
Now...which means actually this month...
A Colony ship needs for the conquering land units (or flying units) and naval units. That's the actually situation.
If one of them are destroyed by a spell or in this case by fire ships, than the Colony ship goes home.
It is possible to use Hydras in case of other naval units. Than the fire ships defenseless and nothing happen. The Colony ship will landing.
And, to make the confusion complete, the new game designer will have a look on the conquer system. Which means it is possible, that this will again change in one year or so.