SLA batteries should never be run until flat, one sure way to ruin a battery that should last any user a good five years at the very least, if looked after.
The discharge cycle on a charger, if you have that facility, is meant to stop the battery from going fully flat and to test it's load bearing capacity. Basically it will tell you if there is a fault in the battery plates and should only be used when you suspect a fault with holding it's charge.
Most people in my experience will lay up a battery over winter and only charge it when they need it. SLA batteries will loose charge over time and then will be ruined, rarely will running the discharge bring them back from the dead. So if you aren't using the battery for a few months put it on charge monthly over the winter. Keeping the battery warm and not in a cold shed/garage / workshop will also help keep them in good condition and prolong their life.
A slow steady charge is better for the battery, rapid chargers work by feeding a higher current through the gel and can cause bubbles to form on the plates inside eventually these tiny bubbles corrode the lead plates that hold the charge.
A newly bought battery will be fully charged and should be put under load by the equipment that is going to use it. Right away if possible install it in your boat and run it, under load (in the boat on the lake ) for thirty minutes or so then charge it up. Charge it every time you get home.
You should have no problems then keeping SLA's for years to come and they will give you many years of sailing..