Why are Drowners and Sirens so easy to kill underwater?

I've been playing The Witcher 3: Wild Hunt on Death March, and with my humble stats it takes me considerable effort to kill a monster on land.

A Level 10 Drowner would take at least 7 slashes of my sword on land, but while in water, one crossbow bolt of 50 damage usually kills it.

Is there any lore related to why monsters are so easy to kill underwater?

Solution 1:

It is unlikely that there is any connection to lore implied here. Instead, your best bet is that it is a gameplay choice for these reasons:

• It gives the regular cross bow another purpose. Because of the low damage it is not useful in regular combat. With explosives bolts (and similar bolts) it is somewhat useful, but not that reliable in combat. On land its main purpose is to bring flying enemies to the ground.
• Getting harassed by multiple enemies can be a pain and I'm sure the developers figured this out, too, so they saw the need for a weapon that can quickly dispose of enemies under water. It may seem unrealistic that almost any underwater enemy is a one-shot kill, but what else could have been used under water? The signs? Maybe. Of course, you have swords, but...
• ... swinging a sword under water in a way that it hurts anything is near impossible, even for a Witcher, so a cross bow is the best tool for the job.