The weapons damage needs fixing
While the smaller dice make it less optimal for a half orc brutal critical or the barbarians ability that adds extra dice to crits the minimum damage is much higher, and having 3 dice to reroll when using the Great Weapon Fighting style is very good. Consider changing to 2d6 or 1d12. If you make improvements to the throwing ability (making it not have disadvantage all the time) consider a 1d10 damage die.
Even though this seems powerful, you can't use it with Dual Wielder or the Dueling fighting style and you still need a feat to make the net not have disadvantage all the time.
This might cause some problems with Great Weapon Master though. GWM is expected to be used with 2 handed weapons as every 2 hander has the heavy tag but it's ability (-5, +10 damage) requires only heavy weapons and not those with the 2 handed tag. Consider removing the "Heavy" tag.
The range should probably read 5/15 as the net, there isn't really a 0ft range in D&D 5e. Even with that adjustment you'll have disadvantage on any throw you make with it.
The feat has problems
While this might seem like it's an always on or never on, you can abuse it by just being underwater yourself. A control water spell to keep yourself on land and underwater would give you advantage all the time.
Consider adding that both you and your target must be fully submerged in water. Although this will make it either: Always on or almost never on. I'm not sure that it's any better but it is less overpowered. Maybe something that doesn't happen as often but can happen anywhere. Attacking into water maybe?
This seems to be okay if you make the above change. Other feats that allow things like this are the Great Weapon Master (You get an extra attack on a crit or kill) and the Pole Arm Master (Extra 1d6 attack)
The GWM ability has a more focused use case than "Whenever you attack" while the PM is an attack at reduced power.
While a net does no damage it does impose a beneficial condition (to the attacker) on the foe, but you only get to do it once (unless you have a plethora of nets on you, even then you'll be limited) and you have to go back to two-handing the trident. On the flip side you do have to forgo all your attacks to throw the net, which might make this underpowered.
The last ability might be underpowered
Having missed the part where it says you have to be attacked by a reach weapon, this part might never get used. If it will effect any weapon my original stance is here:
At level 1 (Variant Human) you can use your reaction to gain +2 AC as with a Shield of Faith spell. There's almost no reason not to do this, and it has the chance of disarming your foe if they're using a reach weapon. By level 13 you've outclassed the Shield spell and you don't need to spend a spell slot. The interesting caveat is that it only works against one creature, I'm not sure how to factor that in.
If you can convince someone to keep Shape Water up at all times you'll always have advantage on attacks. This is probably too powerful and needs adjusting.
All in all, the weapon is very good for any non-orc, non-barbarian 2 handed weapon builds. The feat has some good aspects and some bad ones, but it's far too situational to be useful most of the time. Generally you want to prevent feats from being "Specialty with X weapon". Consider making a feat that applies to all spear-like weapons (Spears, Tridents, Pikes, Javelins) instead.