Water weirds MM entry leaves some questions unanswered, which are ruled on below.
Water weirds can move out of their pool up to a range equal to their length, and attack targets that are in melee range of that farthest point. So a 10' water weird could move up to 10' from the pool, and attack 10' beyond that, for a total threat radius of 2“ around the pool. Water weirds can easily be longer than 10', and this added length would extend their threat radius.1)
Note that the water weird does not have an increased melee distance, but rather that its snake-like form is able to move (as a regular snake monster, but like a tentacle, in the air) out of the pool up to its length. It must still use a move or charge action to extend in this way.
When a water weird successfully grabs a target, it can instantly retract to its pool and begin the drowning process. This movement does not prompt parting attacks because it is part of the special attack.
The water weird is unable to leave the pool at a distance greater than its length unless it takes over a water elemental.
See Drowning.
If the weird is disrupted or otherwise disabled (such as via hold monster), all targets are released.2)
There is no way to free a drowning target from an active water weird by mere strength, other than by disrupting or destroying the weird.
Water weirds are one of the monsters explicitly kept out by protection from evil. The magic prevents the weird from being able to attack the target affected by the protection from evil.3) The water weird cannot bypass the barrier created by protection from evil, but if the weird is already drowning a target, the target gaining protection from evil after the fact will not have any benefit to freeing the victim, because the weird is already within the 1' bubble created by the spell. (For larger protection radii, see Paladin.)
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