Tidal Shell

A protective dome of water flows into existence around and above you and remains stationary for the duration. The spell ends if you leave its area.

One creature of Medium size or two Smaller, can fit inside the dome with you. The spell fails if its area includes a larger creature or too many. 

Inside the shell you gain a +2 AC bonus and any attacks against you have disadvantage. Inside you have advantage on any spell save that requires sight of you and you have resistance against Thunder damage.

Any water spell cast from within gains an extra die of damage. Ice magic can freeze the dome to create total cover. Spells cannot be cast out of the frozen dome without breaking it.

The dome has 20 HP that gets deducted any time it's attacked or penetrated by a spell or magical weapon or ammunition. The shell is immune to any non-physical damage other than Force. The spell ends upon losing all HP.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, you gain an additional 1 AC bonus, and the Shell Starts with 10 HP more. At 4th level and every two after that, the Shell expands by another 5 foot radius, allowing twice the number of creatures to shelter beneath it as well as Large sized creatures.