I think phones can be both waterproof and modular. Going by [1], waterproof gaskets usually come from either glue or silicone. Glue obviously inhibits modularity, but silicone gaskets work fine.
The Galaxy S5 is waterproof, has a rubber gasket over the charging port and a removable battery but not "significantly bulkier" compared to all of the current crop of mid-range/flagship phones - it's actually lighter than the glass-backed ones