Edit: The original question had too many offshoots and was too broad. I hope this new question is better formatted.
I am working on a concept where Earth is unintentionally "invaded" by aliens from an ocean world. Their technology relies entirely on bioengineering and genetic manipulation. The enemy "force" consists of engineered organisms - all entirely organic.
Would modified deep sea dive suits, such as the Navy's ADS, be preferable to traditional submarines against creatures which could potentially latch onto the hull?