Generally, organisms have two possible modes of consumption. They could be mechanical masticators... jaws/teeth/whatsits that mechanically cut or chew. Or, they could chemically pre-digest their prey. In the real world, many organisms rely on both to some degree, though the bulk of the damage will come from one or the other.
Mechanical action tends to require some parity in size, if only because while that army ant's mandibles can take a nip out of you, it only lasts a second before your arm (which has a mass twenty thousand times hers) will smash down Hulk-style and the ant is no more. Even swarming behaviors of many tiny organisms tend to not tackle living prey (exceptions exist but are rare).
Chemical pre-digestion is trickier. They could, in addition to dumping some weird acids/enzymes that start dissolving your skin, also dump any number of other chemicals into the water which would disable their prey. Nerve agents and paralytics and so forth. But, supposing you don't want to do that, it's difficult to imagine a chemical agent that would start dissolving human skin so quickly that within five minutes the victim is dead. Sea water doesn't make it impossible for an acid to dissolve, but its sheer volume tends to dilute things quickly, and tiny organisms have tiny reservoirs anyway. So it'd almost certainly have to be enzymatic in nature.
Perhaps the best approach would be a combination of both... these things attach themselves like barnacles, and drill into your skin. Then, to mitigate dilution, they dump the enzymes directly into these (sealed) wounds. If this were to cause them to release and reposition themselves, the resulting wound might have more surface area for more to attach. Repeat that three times a minute for the next few minutes, and a person might start to look more like a chum bucket. Maybe in such a time frame, they could open up an artery or a vein.
Though, if I wanted to make this sound more plausible, I'd probably back off on the "5 minutes and you're dead" stuff. Sure, with medieval medical science, pulling a person out of the water after 5 minutes might be a death sentence and even within a day or two (even today, if large areas of skin are destroyed, things can be dicey), but they'll still be speaking. Or more likely screaming "kill me now".