Hiccup wore a diving helmet in Race to the Edge, Season 4, in the episode "Dire Straits". He uses it to dive to the sea floor to free the Submaripper by using Changewing acid to melt the chains that bound it. This helmet allowed Hiccup to breathe underwater for an extended period of time. It was developed alongside the Diving Bell.
It's unclear if Hiccup made it himself or if Fishlegs made it, as when we first see it, Fishlegs is handing it to Hiccup shortly before he climbs into the Diving Bell.
Like the Diving Bell, the diving helmet appears to be made of Gronckle Iron and Death Song amber. It also seems to be a modified Berserker helmet. It has traditional Viking horns attached to the top, and a thick leather cowl collar at the neck to keep the water out.
- This is the first time Hiccup has worn Viking horns in the entire Dragons: Race to the Edge series.
- The last time we saw him wear a horned helmet was during the events of Gift of the Night Fury which took place 3-4 years earlier.