Re-Skinning Monster

I have used this method for as long as I can remember doing so back in my AD&D days.

You take a creature, change what it looks like and play it how you want it to be. Make sense?

It's so simple even my son is doing it in his Ruins of Atlant 5e game.
In most of my games I run I start he players off at third level with zero experience, what that means is you play level 3 characters till you get enough experience for fourth level. It only adds two extra sessions to the whole ordeal and makes the characters a little beefier.

The first encounter takes place on a boat as they leave a ship going towards the coast and the type of creature that attacks the group are Grindylow

Imagine half-octopus goblins, how do you make a Grindylow in D&D 5e?

Take the goblin from Monster Manual page 166, you add amphibious (can breath air and water) to the goblin template, change the movement on land to 15ft, keep the sea movement the same as listed in the MM (30ft), lose the short bow (you gave it amphibious, I'm just trying to take a little more from it for balance, you can easily keep the bow), change the scimitar to a short spear, it does the same damage but it piercing now.

I have a sea goblin now!

The tentacles are nothing but thematic for the DM/GM to add to the story and have a cool way for the bad guy to get to know the player characters 😏

Lets do this to a Orc,
Orc is on page 246 of the Monster Mannual, you add amphibious (can breath air and water),we remove the javelin (you gave it amphibious, I'm just trying to take a little more from it for balance, you can easily keep the javelin), change the movement on land to 15ft, keep the sea movement the same as listed in the MM (30ft), change the greataxe to a greatspear spear, it does the same damage but it piercing now.

I have sea Orcs, well they would look differently but I have tougher Grindylow

Now you have the same Grindylow that is a challenge 1 monster instead of the challenge 1/4 for a goblin. It is still small (you can make it medium size if you want to).
 You can do the same with the Ogre template, to make a small Grindylow that can challenge characters of levels that are higher.

You can even use the Giant Octopus from the Monster Manual page 326, you just make the upper half of the body like the picture and bump the INTELLIGENCE SCORE by one point to 5 to match an Ogre INT score.

 The best thing about Re-skinning is that one player you have that always knows what to do against certain monster as they read the monster manual, will be totally blind as you re-skinned everything and they are unaware of what you are doing.