The rules for the Harjel system (TO, p. 288) state: " When operating underwater (see p. 121, TW) or in a vacuum (see Vacuum, p. 54) a unit equipped with the HarJel system is not required to check for hull breaches for any location that features a BattleMech HarJel slot and has any armor remaining in that location." The errata then adds: "However, it does not prevent the need to take crush depth checks (see p. 42), nor does it prevent crushing if it does occur, though a ’Mech with one or more HarJel slots has a +1 TN modifier to any such checks.".
The rules for the Harjel II/III systems (IO, p.88) state: "HarJel repair systems provide a +2 target number modifier to all rolls made when checking the unit’s hull integrity in hostile environments or underwater (see p. 121, TW)."
From this we can conclude that there are 3 possibilities:
1. There's no mistake in any of the statements, and the Harjel system is superior to Harjel II/III systems in regards to the hull breaches. (Counter-intuitive though.)
2. The entry for the Harjel system incorrect, and the hull breach checks should be made at an any depth with +1 TN modifier.
3. The entry for Harjel II/III systems is incorrect, hull breach checks should not be made under normal conditions, and +2 TN modifier is for the hull integrity checks at extreme depths (TO, p.42).