When you create the tentacle, you can make a melee spell attack against one creature within 10 feet of it. On a hit, the target takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn.

Vampiric Touch seems obvious.

Does Shadow Blade count as a melee spell?

What about Booming Blade and Green-Flame Blade?

Blade of Disaster?

Are there any Cantrips I can use with Tentacle of the Deep?

Other spells?

You can’t cast a spell through it.

Melee spell attack means you use that as your to hit modifier as opposed to your weapon attack modifier. Basically your casting stat bonus plus proficiency bonus

The tentacle is the melee spell attack. You can't cast a spell through it.

Tentacle of the Deep is treated as a melee spell attack (once the tentacle is created). Is this what you mean?

Quote from AstronautDancer >>

Tentacle of the Deep is treated as a melee spell attack (once the tentacle is created). Is this what you mean?

 Basically, there is a difference between a "spell attack" and casting a spell. Casting a spell may require a melee spell attack (like shocking grasp), but non-spells can also require melee spell attacks as tentacle of the deep does.

Basically, it just means it is magical and not a weapon attack.