Load testers are more designed to see if the battery can support the load of a starter motor, or how well it holds its CCA rating.
Deep cycle batteries have much less CCA to begin with, compared to a starting battery.
A Hydrometer is a tool to see how well any given charger is doing. Unless a battery manufacturer posts a SpecificGravity/State of charge chart, a hydrometer reading is only useful for determining when a battery is fully charged and tailoring ones charging voltages and durations to make the battery happy. It is a battery polygraph. Most automatic charging sources are dimwitted liars with awe inspiring marketing that the lemmings follow over the cliff.
A load test on a deep cycle battery is to place a 5 amp load on a 100 amp hour battery, and see how close to 20 hours it can provide that 5 amps before voltage falls below 10.5v.The test itself is abusive to the battery.
Amp hour counters are good for noticing trends and tendencies. When one sees 30 amp hours from full, but then notices the voltage is reading 0.1 or 0.2 volts less than what they are used to seeing for 30AH from full, then once investigates further, with the first tool being the hydrometer, and the second tool, usually a charging source capable of applying Equalization voltages. The third tool is to begin saving cash for a new battery for when tool 1 proves tool 2 was too little too late.
If one wants to guestimate how long any given starting battery will still be able to crank the starter motor, then a battery tester has merit, but on Deep cycles, the merit is much much less.