Not to be confused with ordinary merfolk, merrows are larger-than-humanoid monstrosities, the descendents of merfolk warped by demonic influence in the ancient past. Since they’ve bred and survived since then, we can consider them evolved creatures despite their supernatural origin.
Merrows are water-dwelling creatures, drawn to coastal areas with a lot of marine traffic, where they prey on anyone and anything weaker than themselves. While they can breathe both air and water, they flounder about at a pitiable 10 feet per turn on land, but in water, they swim at a brisk 40. Their exceptional Strength and very high Constitution place them in the brute category, eager to get up close and personal with their prospective victims. They’re not that bright, but they can tell when they’re getting beaten. They also have darkvision, so the hours from twilight to dawn are particularly dangerous times to be messing about in boats where merrows roam.
Most humanoids are strongest on land and weak in the water; merrows are strongest in the water and weak on land. It’s a given, therefore, that when merrows attack, their first and foremost goal is to pull their opponents into the water.
Their harpoon attack gives them the means to do this. When a merrow lands a harpoon hit against any Huge or smaller creature, the two must make opposing Strength checks—and the merrow, with its Strength of 18, has a good chance of coming out on top. If the merrow wins, it yanks its opponent up to 20 feet toward itself. (The harpoon’s melee reach is only 5 feet, but the merrow can throw it up to 20 feet without disadvantage, up to 60 feet with.) Since the merrow is in the water and the target presumably isn’t, that means . . . well, now the target is in the water.
Aquatic combat is a mess for anyone without the innate or magically endowed ability to swim. First, any such combatant has disadvantage on melee attacks unless he or she is wielding a thrusting weapon: a dagger, shortsword, javelin, spear or trident. Ranged weapon attacks at normal range are made with disadvantage, and ranged weapon attacks at long range automatically miss. And a character with a Constitution of 11 or lower can hold his or her breath for only 30 seconds—five combat rounds. (Characters with higher Constitutions are good for the duration of virtually any combat encounter.)
And let’s talk about grappling. It’s a contest between the grappler’s Strength (or Athletics skill) and the grapplee’s Strength (Athletics) or Dexterity (Acrobatics), which, again, is probably in the merrow’s favor. A grappled creature’s speed, including its swimming speed, becomes 0. Even if he or she can wriggle free, a creature without a specific swimming speed can move through water at only half normal speed, and in rough water (such as merrows surely like to inhabit), he or she needs to make a Strength (Athletics) check to do even that. So once a merrow has an opponent in the water, that opponent’s likely to remain there awhile.
And let’s talk about visibility! On a bright, sunny day, you can see through a few feet of clear, rippling water. But it’s hard to see through any depth of choppy water in the best of conditions, let alone in dim light or darkness. So while merrows lack proficiency in Stealth, they can often remain unseen simply by staying underwater. As a dungeon master, I’d rule that choppy water is always one level more obscured than the open air above it, so a merrow underwater during daylight hours is lightly obscured, and it’s heavily obscured at any other time. A heavily obscured creature is unseen by definition; a passive Perception check to spot a lightly obscured creature comes with a −5 penalty.
So the merrow’s strategy begins to take shape. It stalks a boat, harpoon in hand, making a token Dexterity check for stealth purposes, just to give it a shot at a surprise round (multiple merrows attack from multiple directions, with the intent of pulling their targets apart from one another). When it closes to a distance of 15 or 20 feet, the merrow pops up out of the water, hopefully unnoticed, and hurls the harpoon with the advantage of being an unseen attacker. If it hits, it tries to haul its target into the water alongside it, and if it succeeds at that, it follows up with a bite.
What the merrow does next depends on its target, and also on whether it missed with its initial harpoon attack. If it did miss, rather than retrieve or reel in its harpoon, it switches to claw-bite rather than harpoon-bite. If it hit, however, and if the target hasn’t gotten away between turns, it takes its harpoon by the shaft and continues to stab its target while also biting.
A weak swimmer probably won’t be able to make it back to the relative safety of his or her watercraft, and the merrow will continue to use its double melee Multiattack. But a stronger swimmer may be able to cover some distance, and if so—let’s say, if he or she manages to swim 15 feet or more away from the merrow—then it forgoes its Multiattack in favor of a grapple attack, and if it succeeds, it hauls its target 20 feet back in the other direction (40 feet if its target is Small). Then, on its next turn, while holding its foe grappled (assuming he or she hasn’t gotten away), it resumes Multiattacking.
Also, it goes without saying that the merrow uses its reaction to make an opportunity attack against an opponent who manages to swim out of its melee reach, using whichever available attack does the most damage, i.e., the harpoon if it has it in hand, its claws otherwise.
A seriously injured merrow (reduced to 18 hp or fewer) will think better of the encounter and retreat, using the Disengage action (reflecting instinct and the advantage of living in the water, not training or discipline) to submerge and swim away, and releasing any grappled opponent. Any merrow whose opponents somehow manage to drag it out of the water, regardless of how much damage it may have taken, will Dash back toward it—it’s not about to try to win a fight on solid ground, however strong it may be.