A hydrolocked engine in your 2021 Honda Pilot means water entered the combustion chambers and caused internal damage when the engine tried to compress the incompressible water. This is serious damage requiring professional assessment to determine if the engine can be repaired or needs replacement.
How Hydrolock Happens
Water enters through the air intake when driving through deep water. The pistons attempt to compress the water on the compression stroke. Since water doesn't compress, something must give - typically connecting rods bend, pistons crack, or the crankshaft is damaged. The damage occurs in fractions of a second.
Assessing Hydrolock Damage
Remove spark plugs and inspect cylinders for water. Attempt to turn the engine by hand - difficulty indicates internal damage. Perform a compression test to identify damaged cylinders. In severe cases, the engine won't rotate at all due to bent rods jamming against the block.
Repair Options
Minor hydrolock caught before severe damage may only need water evacuation and oil change. Moderate damage may require connecting rod and bearing replacement - a major repair. Severe damage usually means engine replacement is more economical than repair. The Pilot's 3.5L V6 replacement or rebuild is a significant expense.
Prevention for the Future
Never drive through water of unknown depth. Turn around if water is above the bottom of your bumper. If you must cross water, go slowly to minimize the bow wave that pushes water up into the intake. Standing water after heavy rain can be deeper than it appears.