Yes, the first 2 siphons in the Groin Canyon are mandatory. If the water level in the canyon is low, the siphons are "defused", that means we do not need to put our head under the water. On the other hand, when the water levels are high, it is necessary to dive in order to cross.
The passage of the siphons is very wide, like a door size. It is therefore not possible to get stuck in there.
They are not long: each siphon is less than 20cm long. This is very easy to cross: it is not harder than swimming under the line in the swimming pool.