Hi Ian
What boat are you using this in and what's the motor setup.
A SLA battery will only give you its stated amp hours if it is discharged at a rate of 0.05C so to get 7 amp hours out of your battery it will need to be discharged at 0.35 amps
If you discharge it at a higher rate you will get less amp hours form it At 1C 7amps it will only work for about 35mins and give you just over half its stated capacity. If you discharge it at 2C 14amps if will last about 12mins and give you less than half of its stated Capacity. You can find all the facts here
http://www.yuasabatteries.com/industrial.php just look for the data sheet on your size of battery at the bottom of the page.
To test what your battery is still capable of holding then you need to find someone with a charger that will discharge as well as charge and that has a display to tell you what it has taken out of the battery as well as what it has put in.
here are a few examples
http://www.component-shop.co.uk/html/body_radio_control1.html look at the two Turnigy half way down.
http://www.graupner.de/en/products/843d18cf-1eaa-4aa4-a3d0-fb05359d6f7f/6468/product.aspx I have the Ultramat 15 and when I charge the 7Amp 12 Volt SLA's I use in my Arun they only take between 3.5 to 4 amps to be fully recharged and I know that at full throttle the Arun's motors are pulling 12 amps but its not run continuously at full throttle when its on the water so I can get 25 mins running time out of a battery.
If your battery is on its way out there are a couple of options. The chepest is to buy another SLA if the old one worked well in your boat and the running time was long enough for your needs. If reducing the weight would be an advantage then you could look at a 5000mAh sub C NiMH pack. This would be less than a third the weight of your SLA and give you the same running time. The disadvantage is that it would cost more and you would also need a new Charger.
If you want a longer running time then you could look for a NiMH pack of 9000 mAh D cells or 13000mAh f Cells both of these would still weigh less than a 7A 12V SLA disadvantage is the cost and the need for a new charger. I have a F cell pack from my boat and it gives me more than 3 times the running time as the old SLA and of course there is also the LiPo option.