A Water-Hybrid must make up for a lack of physical prowess with an exceptional wit.

+2 INT +1 WIS

You transform a portion of your body allowing an attack to pass through, leaving you unharmed.

When you are hit by an attack, you can use a reaction to roll 1d4 and add the total to your AC.

Gain resistance to fire and poison. Immunity to poisoned status effect.

Your skin is cool to the touch and moist, fire needs to burn a little hotter to get through. Toxins in your body can be shunted out at will.

As a bonus action you can fully transform your body into water,
you can retain the shape of your physical appearance or
change into any shape your weight in water could look like.

You lose the transformation when unconscious.

While Transformed:

You have immunity to all nonmagical bludgeoning, piercing and slashing attacks.
You cannot be knocked prone.

You have disadvantage on all melee and ranged attacks.
It is hard to wield weapons or throw a solid punch when "being solid" requires focus on it's own.

You cannot speak while transformed without using an action specifically just to do so.
(Can't cast spells requiring verbal)

You are susceptible to certain spells, such as create/destroy water, and shape water.
Similarly ice spells may slow you down or even freeze you completely (up to DM) 

Your movement speed becomes 5 feet unless there is significant water within 5 feet of you,
such as an inch deep pool, or heavy rain.
When in water, you can use a bonus action to dash. 

You can make a ranged spell attack.
You shoot a jet of water 60ft at the target dealing 1d8+Proficiency on hit.
 The spell creates more than one jet when you reach higher levels:
two jets at 5th level, three jets at 11th level, and four jets at 17th level.
You can direct the jets at the same target or at different ones.
Make a separate attack roll for each jet.
This is considered a cantrip.

You can use an action to occupy the same space as another creature within 5 feet of you
and attempt to restrain them by covering them with your form.
They must be the same or less than your size.
They must make a STR save against your spell save DC or be considered restrained.
A creature without a swimming speed is also considered underwater.
If the creature is hit by an attack that must travel through you, both of you take half the damage it would have done.
You can use another action on a creature restrained in this way to attempt to crush them using your liquid body.
The target must make a CON save against your spell save DC.
On a failed save the target takes 2d10 bludgeoning damage and is stunned.
On a success they take half and are not stunned.
This is considered a level 1 spell. Add 1d10 per spell level.  

Being Half-Water you feel the need for water differently.
You feel thirsty much more often, despite not "requiring"  water as others do.
You can survive for up to a week as opposed to the usual 3 days without water.

Should you go even a day without water you would likely notice your body thinning rapidly.
Without regular water intake your body loses mass.
You begin to look more and more emaciated as time passes and you rapidly lose weight.

This process is accelerated in hotter temperatures and in direct sunlight.
Shorting your expected survival duration greatly.
Should you go without water into a hot area while in direct sunlight,
you would lose mass very quickly and possibly even die in just a days time.

Many Water-Hybrids often carry copious amounts of water on them for this reason.
Some using large gourds or even magical canteens.

You can breather underwater and you have a swimming speed equal to your walking speed.

You do not need to breathe so long as you have mass.

You can fill your lungs with water and use the oxygen in it alone to live.

You may still choose to breathe normally in order to retain mass.