Okay so my players have to cross an underground lake to get to an island. There is a boat available to take them but the boatman is a skeleton and the whole thing reaks of trap. If they try to swim, fly or do anything other than take the ferry they will be attacked by the lakes guardian. It's a Half-Dragon Kraken. It can fly, swim and crawl! The question I pose, is what kind of dragon should it be? There are many options! The creating a half dragon section only lists a couple base dragons but now I hear there are space dragons and such. So what is the coolest dragon breath weapon out there?
Well for your criteria I'd immediately suggest something that's evil (or at least non-good) and breathes water. Black, Brine, and Green all fit the bill.
Imperial (Sea) Dragon has a hot steam (cone of fire damage) breath weapon, which can be altered to instead be a pressurized jet of water (line of bludgeoning damage) - a nice surprise after the group's caster sees the steam breath and casts Resist Energy (Fire).
Which would be useful if Half-Dragons got their breath weapon more than once a day.
Also, Half-Dragons don't have the ability to use the variant breath of the related Dragons.
Which is a bless, in the case of the Void Dragon, since its variant breath is a mechanical mess.
Teleporting would result in being attacked as well. The Island is small and will not shelter them from attack. Mist from the lake obscures view. In theory they could scry or blind jump but they know better than to teleport. Last time they did that they ended up in a vacuum sealed tomb. They best part of that was that it was hard use thr verbal component of teleport without air to get back out.
Or the time they teleported underwater with deeper darkness and shadows. The ice cold water masked the cold of the shadow's touch. Took em awhile to figure out why they were losing strength. Couldn't feel them, see them, hear them, smell them or taste them. Just round after round of fort saves.