Just to be charitable to the user who’s arguing with you, I suppose it’s maybe possible they’ve seen pictures of methane clathrate deposits on the seafloor, which does look a lot like ice and is sometimes even (erroneously) referred to as “methane ice”. I could see how someone might see photos of a methane cold seep in NatGeo or something, see people on boats topside holding samples of what looks like ice, and possibly getting confused over the text referring to it as an ice-like substance at the bottom of the ocean.

However, methane clathrate is obviously not actually ice (or at least it’s not usually classed as one of the forms of water ice and is really kind of its own thing chemically), and I might be assuming too much good faith here . In any event, this other user needs to admit that they’re probably not going to win an argument about basic physical properties of seawater with an actual expert on the physical properties of oceans.