Towards the end of training, there are only about eight or nine recruits in the squad bay; this would be an extremely small and unrealistic number, given that most recruit platoons start with around thirty privates and typically only loss a handful to attrition.
French is outed as a homosexual because others see him get an erection in the shower; however, this really isn't that unusual and is typically caused by nocturnal penile tumescence (also known as NPT and commonly known as "morning wood") especially if taking a shower right after getting up in the morning.
The Senior Drill Instructor would not in any way be involved in swim training, and certainly wouldn't be able to attempt to drown a recruit and get away with it. Swim personnel typically are Navy instructors and also have a Corpsman on duty. Trying to drown a recruit in the pool would therefore be seen by a lot of people are reported immediately.