Thorough check of wheel and bearing. If you dunk the trailer, you should change the bearings once a year. Use a high quality, high temp grease which should state made for trailer bearings. You do not want to use an inexpensive grease. Most inexpensive grease will turn to mud. Grease tubes 10oz will range in price from 1.97 to 9.97. Do not think you are saving money by using the less expensive item. Read the Cannister.
If you want a quik check of bearings put a jack under the trailer. Grab the tire and attempt to rock the wheel back and forth. If the wheel has play you need to retorque the spindle nut and add grease.
High temp grease will stay a lubricant under temp and excessive wear. The problem with dunking a trailer is the bearings will want to rust in any exposed area or from the mix of salt water and grease.