My players fought a water weird last night, and its attack as written on page 299 of the Monster Manual is rather complex. First, you make a roll as normal and do bludgeoning damage.
If the target is Medium or smaller, it's grappled so its speed becomes 0 and it has all the other effects of that condition. The target is also pulled 5 feet towards the water weird.
While the grapple is in effect, the target is also restrained. The effects of that condition are the target's speed becomes 0, which is redundant with being grappled, the creature has disadvantage to hit, but advantage to be hit, and the creature has disadvantage on Dexterity saves. Also, the water weird's grapple includes the notation "…(escape DC 13)…". However, the rules for escaping a grapple are that a contested check must be made, not that a specific DC needs to be reached.
The next part is where the problem comes in. The text says "…the water weird tries to drown it, and the water weird can't constrict another target." From what I can find in the PHB and DMG, drowning uses the suffocation rules. Those state that a creature can hold its breath for 1 + Con modifier minutes (minimum of 30 seconds), then can survive an additional number of rounds equal to its Constitution modifier before it starts dying.
So, a water weird facing an adventuring party hits one of the players and grapples/restrains/begins drowning it. That player has, at the very least, 5 combat rounds (the minimum 30 seconds) to try to escape. However, during that time, the water weird has no other action to take because it can't constrict another target, and the rest of the party can hit it with whatever they want.
Am I understanding this correctly, that the water weird (if it hits) is pretty much a one-shot monster? What's going on with the specific DC to escape the grapple, rather than using the standard rules?