EPDM rubber belongs to the thermoset elastomers (rubber) classification, while PTFE belongs to the thermoplastics. Please note that the two materials have significantly dissimilar densities.
EPDM rubber is a thermoset elastomer or rubber material. It has a fairly low density and the highest heat capacity relative to other rubber materials.
PTFE is a fluoroplastic (fluorine-based thermoplastic). It can have a fairly high density among thermoplastics. In addition, it can have a moderately high ductility and a fairly low tensile strength.
EPDM is softer and makes a better sealing material (conforms better to rough surface finishes). Teflon is stiffer and slicker, which might hold up better in a dynamic or abrasive application. Both could care less about sea water from a chemical attack/corrosion standpoint.
The decomposition temperature of PTFE is >490°C (>910°F) and the main decomposition products are hydrogen flouride gas (extremely corrosive) and carbon dioxide. It is unsuitable for use where it will come into contact with acetone or hot alkaline applications.
If you want the parts to seal use EPDM or Natural rubber, the PTFE material will not seal well at all. It is harder, yet more durable & certainly more costly but it most likely will not seal as well as an elastomer.