1: Sea Monsters
For the love of gods, why aren't sea serpents used more in games like D&D? I don't own the 5e Monster Manual, but I am pretty sure there is roughly one kind of giant sea monster: the kraken.
Don't get me wrong, I love me some krakens. I've used them. I've had them attack ships. I've had them attack subterranean Aboleth cadres in Pacific Rim-type cave-lake encounters. Super fun.
But there is SO MUCH MORE sea monster mythology out there to be used. Hell, you can throw anything underwater and make it cooler.
How about aquatic hobgoblins? No, not sahuagin (or whatever they are called). As soon as you put a fish face on something, it looks stupid. Big dumb eyes and wide toothless mouths. That is not scary. Why not more aquatic mammals or reptiles that need to breath air, but not very often.
Like a crocodile. Sweet mother of Thor crocodiles are scary. Stealth killers that get you at the watering hole?
Also: water. No need to add all those ridiculous terrain changes and traps and whatnot. Deep water is scary enough as is.
2: Battle Formations
Smart enemies fight in formation. Maybe not a straight line, or a tortuga, or whatever that roman thing was with all the shields and spears and such, but they know where they are supposed to be to make things as bad as possible for the PCs.
This means you can't just sprinkle the hobgoblins on the random forest map and expect it to end up like a real fight. And I don't mean "real" as in "like the real world," I mean real as in "dramatic and requiring strategy."
Let's see some regrouping and battle chants and (for the bigger battles) war drums and horns and such. Opportunity attacks make this needlessly difficult, which is why I hate them.
But at least consider allowing the monsters or PCs to pull back and reestablish positions on higher ground or better terms.