FORD 1999 E-450 - Manual (Page 122)

Replace the rear axle lubricant anytime the axle has been submerged in
water. Rear axle lubricant quantities are not to be checked or changed
unless a leak is suspected or repair required.
Disconnect the wiring to the trailer before backing the trailer into the
water. Reconnect the wiring to the trailer after the trailer is removed
from the water.
Driving
122