Most of the SM fluff has them sinking.
Brotherhood of the snake, main mentions could just walk to unknown primarch statue with his armor but that would break the tradition. The fluff you had said that they sank to the bottom and fight as norm. Soul Drinkers book, 1st, some SM got knocked off a boat and sank, Serp said that if they were lucky they would get crushed otherwise they would either suffocate or starve to death. Fulgrim (HH) Mentioned similar on Lyr (opening planet)
Seems incorrect, especially the suffocating part, they have a third lung that can use water afterall, hell we have stories of them hunting sea creatures with and without armour without sinking.
probably just another mistake, different authors saying different things