There's no mention of water breaking line of effect, so it's legality is completely up to the GM.
We have a situation where we are facing a black dragon that is ~15' under murky water (to gain total concealment). It breathes from that depth and hits a PC that is standing on land with acid. Is the attack entirely legal?
Keep in mind that the PCs will want to rain down AoEs and other ranged attacks into the depths as well.
Is there any rule that arrows cannot pierce that water, for example?
As a DM, you will need to decide whether water serves as a negligible, soft, or hard barrier, and then stick to your choice consistently.
Seconded for truth!As a DM, you will need to decide whether water serves as a negligible, soft, or hard barrier, and then stick to your choice consistently.