Yeah thats makes a good point - perhaps to prevent it being OP keep air creatures immune.
The most similar card would be plague - that causes poison damage that causes 1 damage per turn to all enemy creatures on the field.
Differences:
- Plague cannot be stopped as it is a magic card, Flood is a permanent and so can be targeted
- Plague doesnt affect new creatures played, while flood would
- Flood affects both sides of the field
Fire shield has a similar principle - every attacking creatures loses 1 hp each turn. This is also a permanent that has similar effects.
Because of these, I would suggest that this is reasonable without being too OP if the following conditions are met:
- THe cost of teh card has to increase - perhaps 5
- Each turn 2
should be absorbed (instead of just one)