Deep-sea diving and Skin diving

Skin diving and deep-sea diving are semantically related In some cases you can replace phrase "Skin diving" with "Deep-sea diving", this phrases are similar.

Skin diving noun - Underwater swimming without any more breathing equipment than a snorkel.

Deep-sea diving and skin diving are semantically related in underwater swimming topic. You can use "Deep-sea diving" instead a noun phrase "Skin diving".