Generally, a lubricating grease designed for automotive wheel bearings is sufficient. Look for the NLGI GC or GC-LB service mark on the product label. If the boat trailer is used in salt water, a specially formulated product with salt water corrosion protection is recommended. Such products are typically available from boating and marine specialty stores. Boat trailer wheel bearings may need more frequent relubrication due the immersion of the bearings to water. Follow the relubrication recommendations of the trailer manufacturer.