Is this only to use in a pool?

No you can use this as a personal lounger if you don't have a pool.

Would be perfect for smaller child. Deep enough for just enough water.

Is there a difference in the depth of the water from XL and XXL?

There is no difference in water depth between XL and XXL. The only difference between XL and XXL is size.

Does the pump come with it?

The pump is not included,the product includes a float, a pillow, and 4 patch pads

Can you put water in this while it’s floating?

The XL float can accommodate 1 ~ 2 adults, about 200 pounds. The XXL float can accommodate 1 ~ 3 adults, and its bearing capacity is about 350 pounds, so you can add water if you only laying for 1 or 2 people.