If it is indeed a 30 Amp heater, which it certainly seems to be and at 240 volts then 10/2 is what you need. As 10 gauge (copper) wire is rated at 30 Amps. Yes you will need a two-pole, 30 Amp breaker so make sure you have room in your outside service panel for one. Under 100 feet you should be fine and won't need to step up the wire size. Check local electrical codes to to determine whether or not you will be required to have a safety disconnect at the unit. Typically required if the unit is not in line-of-sight with the breaker panel. It may require three-conductor wiring, two hot, one neutral and a ground if it has any lighting or LED displays on it.
You should be able to tell in the box where connections are to be made. There will be studs for the wires to attach, usually with lugs. There will be two or three, one with a red wire, one with a black wire(typical two conductor setup) and/or a third with a white wire(usually in the middle) with a fourth marked with a green or bare wire.(ground) Often the ground will be somewhere else, typically called an equipment bond as it is typically found mounted to the casing of equipment, even in the box for connections. Usually there is a green screw or lug, this is the tell-tale sign for ground.
Hope this helps.