Normally the soft dry sand is where vehicles have problems. If in any doubt ask local offshore anglers about the condition of the beach. Watch a few of them go first and take note of where they are launching on the beach. Watch how they drive over the dry sand. In the Landrover I use low range third or fourth if the sand is hard, but select low second if it's soft and keep the revs high.
Soft sand causes a surprising amount of resistance.
Even big diesels loose revs quickly when they hit a soft patch, especially when towing a heavy boat. Ideally the trailer's track should be the same width as the tow vehicle's as it causes less drag and rolls on compacted sand. However unless the trailer is custom built it's generally pot luck whether this is the case. Besides, it's not essential, just handy. If you do come to a grinding halt then don't panic.