To mitigate marine pollution caused by EPS buoys, the Ministry of Oceans and Fisheries of Korea is promoting a policy to support the supply of eco-friendly buoys. An eco-friendly buoy manufactured via the blow injection molding of HDPE must maintain a buoyancy level of 80% or higher compared with the initial buoyancy after subjected to a seawater pressure test. In this study, the change in buoyancy caused by deformation resulting from a seawater pressure of 0.05 MPa subjected upon eco-friendly buoys manufactured via recycling is observed. To predict the change in buoyancy, the change in the physical properties of HDPE due to recycling is measured via a tensile test, and a nonlinear structural analysis is performed using the measured properties. Predictions via experiment and analysis indicate that the change in buoyancy is 90.5%, which agrees well with the experimental result.