1) You have two Water-types.
2) Together they are weak to Grass, Electric, Ground, and Psychic.
3) Regice covers Grass and Ground, while having respectable bulk. It does have four weaknesses, however.
4) Regirock covers none of these and resists none of these.
5) Reegisteel covers none of these, however it resists Psychic, Grass, and takes neutral from Electric. It is weak to Ground.
In conclusion, it would probably be best to replace one of your Water types with Registeel.