Well, you can call it a cargo bay with a 100 ton capicity and add a door for the mech to depart. But, with my own albiet quick experiement on HeavyMetal Vee, you can only get two on before it becomes so over weight that it is unable to move. You'll need an XL engine, and you'll only have about 30 tons for armor and equipment. Speed is 1 / 2. Honestly, the 'Mechs can walk faster under the water.
Here's the basics, if you like.
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[legal]
BattleTech Vehicle Technical Readout
VALIDATED
Type/Model: Mech Carrier Boat
Tech: Inner Sphere / 3060
Config: Surface Ship
Rules: Level 2, Standard design
Mass: 300 tons
Power Plant: 270 GM XL Fusion
Cruise Speed: 10.8 km/h
Maximum Speed: 21.6 km/h
Armor Type: Standard
Armament: None
Manufacturer: (Unknown)
Location: (Unknown)
Communications System: (Unknown)
Targeting & Tracking System: (Unknown)
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Type/Model: Mech Carrier Boat
Mass: 300 tons
Equipment: Items Mass
Int. Struct.: 120 pts Standard 0 30.00
Engine: 270 XL Fusion 2 7.50
Shielding & Transmission Equipment: 0 4.00
Cruise MP: 1
Flank MP: 2
Heat Sinks: 10 Single 0 .00
Cockpit & Controls: 0 15.00
Crew: 20 Members 0 .00
Armor Factor: 0 pts Standard 0 .00
Internal Armor
Structure Value
Front: 30 0
Left / Right Sides: 30 0/0
Rear: 30 0
Weapons and Equipment Loc Heat Ammo Items Mass
--------------------------------------------------------
'Mech Bay Capacity Body 1 100.00
'Mech Bay Capacity Body 1 100.00
--------------------------------------------------------
TOTALS: 0 4 256.50
Items & Tons Left: 61 43.50
Calculated Factors:
Total Cost: 55,125,000 C-Bills
Battle Value 2: 206 (old BV = 30)
Cost per BV: 267,597.09
Weapon Value: 0 / 0 (Ratio = .00 / .00)
Damage Factors: SRDmg = 0; MRDmg = 0; LRDmg = 0
BattleForce2: MP: 1N, Armor/Structure: 0 / 4
Damage PB/M/L: -/-/-, Overheat: 0
Class: NS; Point Value: 2