I need to buy a small 12v battery to power a bilge pump for my boat and i dont know about the ratings etc.Amp Hours??

The bilge pump i have is 12v and has an amp draw of 2.1 @ 12v, so bearing in mind i wont be charging this battery all the time what battery would i need to be sufficent to use it for a few outings with out recharging?

Copy an pasted this from maplin page, the ones they have:

7Ah L/Acid Bat 12V 26 in Stock £24.99
4Ah Ld Acid Bat 12V ~28+ days £19.98
2.8Ah L/Acid Bat 12V 98 in Stock £18.99
2.1Ah L/Acid Bat 12V 121 in Stock £17.99
1.2Ah L/Acid Bat 12V 26 in Stock £16.99

Any help much appreciated