PES (polyethersulfone) braided fiber cordage might be the perfect intersection of desirable characteristics for cave diving guideline: the elasticity of nylon plus the ultra strength of UHMWPE without being buoyant, stiff or slippery.

A {656 ft | 200 m} spool of 1.6mm PES (polyethersulfone) cordage developed specifically for cave diving. This material is often compared to UHMWPE cordage, such as those from Spectra® or Dyneema®, though PES is slightly stronger than UHMWPE as well being heavier. Unlike UHMWPE, this material is negatively buoyant and holds knots well (its not 'slippery'), but isn't considered spliceable. In archaic twine sizing this falls somewhere between #18 (~ 1.5mm) and #21(~ 1.7mm); thinner but many times stronger than nylon #24, it also maintains it's shape better than nylon and you can load plenty on the reel. This line has a little less stretch than nylon #24 but when wet Divesoft tests found a {10 ft | 3 m} length of their PES line to have about {2-4 in |50-100 mm} of stretch when simulating tension to mimic a tie-off. Here is a comparison of UHMWPE and PES characteristics.