We already have Dark Eldar codex and while the review is being prepared, let's look at some ideas. You'll probably need much transport, so check some interesting conversions found on the Internet.
Nobody likes completely similar boats, right? Moreover, Dark Eldar army doesn't tend to be monotonous. You can make your Riders more interesting by using plastic additions...
...or just by changing places of some parts of the set.
And you also can go in for such huge conversion which turns usual Raider into a dragon.
Or something like this, for Coven units. You will also have a lot of them.
If you are not sure what you need more - Raider or Venom, you can make something which reminds of both kinds of transport.
In general, you can add diversity by using bits and leftovers from any other Eldar sets.
And you can logically assume that if the big boats have sails, the smaller ones will also want.
And even Hellions are much more fun with the sails.