f you really want to build a hydromancer/cyromancer focus on frost/water spells and pick a class that allows you access to the most number of those spells.
Is not exactly "hydromancer", Lurker of the Deep has a lot of cool abilities not seem in other classes, also we don't even know if Larian will for eg, pick sorcerer of draconic bloodline and restrict it to only "fire/red" like many other games did.
Pathfinder Kingmaker was the first CRPG which allowed me to be a sorcerer of silver draconic bloodline.I can't judge DOS2, honestly i din't liked much dos2. But IMO the best game to be a water magician is Gothic 3. Gothic 2 and 1 are better games but limits far more your choices. As i've said, a lot of games limits cold based stuff.
In BG classes are much more important so focussing your damage type to one could leave you at a disadvantage
I know and that is exactly why i love Warlocks on 5e. They are very strong in some situations and very weak in others.
Picking DDO for eg, i had a lot of trouble against undeads. Helmed Horror which
heals from force and cold are by far the worst enemy to my char. The unique skill which i have against then is black tentacles which the devs putted 20 sec cooldown, ddo is a decent game but could be great if it had spell slots instead of cooldowns and din't hate stat inflation...
Hypothetically what would your Warlock with the Lurker in the Deep do if it was attacked by a water elemental, and what would my Fiend Warlock do if they encountered a hostile fire elemental?
Water Elementals aren't immune to cold. On 5e, immunities are far more rare than on 3.5e, and imunity against cold is not common and even if is, i have eldritch blast,
a pure force damage. I don't even think that force dragons exists on 5e...
Also, we are talking about a party game, having 1/4 of your party useless in a encounter is a disadvantage, but is not game breaking.
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edit : checked and Marids are only resistant to cold on 5e, not immune
http://dnd-5e.herokuapp.com/monsters/marid 
