Recreational diving in the context of "Liverpool Bay"

A diving helmet is a rigid head enclosure with a breathing gas supply used in underwater diving. They are worn mainly by professional divers engaged in surface-supplied diving, though some models can be used with scuba equipment. The upper part of the helmet, known colloquially as the hat or bonnet, may be sealed directly to the diver using a neck dam, connected to a diving suit by a lower part, known as a breastplate, or corselet, depending on regional language preferences, or simply rest on the diver's shoulders, with an open bottom, for shallow water use.

The helmet isolates the diver's head from the water, allows the diver to see clearly underwater, provides the diver with breathing gas, protects the diver's head when doing heavy or dangerous work, and usually provides voice communications with the surface (and possibly other divers). If a helmeted diver becomes unconscious but is still breathing, most helmets will remain in place and continue to deliver breathing gas until the diver can be rescued. In contrast, the scuba regulator used by recreational divers must be held in the mouth by bite grips, and it can fall out of an unconscious diver's mouth and result in drowning.

Diving equipment, or underwater diving equipment, is equipment used by underwater divers to make diving activities possible, easier, safer and/or more comfortable. This may be equipment primarily intended for this purpose, or equipment intended for other purposes which is found to be suitable for diving use.

The fundamental item of diving equipment used by divers other than freedivers, is underwater breathing apparatus, such as scuba equipment, and surface-supplied diving equipment, but there are other important items of equipment that make diving safer, more convenient or more efficient. Diving equipment used by recreational scuba divers, also known as scuba gear, is mostly personal equipment carried by the diver, but professional divers, particularly when operating in the surface supplied or saturation mode, use a large amount of support equipment not carried by the diver.

On 21 June 1919, shortly after the end of the First World War, the Imperial German Navy's High Seas Fleet was scuttled by its sailors while held off the harbour of the British Royal Navy base at Scapa Flow, in the Orkney Islands of Scotland. The fleet was interned there under the terms of the Armistice of 11 November 1918 while negotiations took place over its fate. Fearing that either the British would seize the ships unilaterally or the German government at the time might reject the Treaty of Versailles and resume the war effort (in which case the ships could be used against Germany), Admiral Ludwig von Reuter decided to scuttle the fleet.

Intervening British guard ships were able to beach some of the ships, but 52 of the 74 interned vessels sank. Many of the wrecks were salvaged over the next two decades and were towed away for scrapping. Those that remain are popular diving sites and a source of low-background steel.

Sinking ships for wreck diving sites is the practice of scuttling old ships to produce artificial reefs suitable for wreck diving, to benefit from commercial revenues from recreational diving of the shipwreck, or to produce a diver training site.

To avoid undesirable ecological impact, and to maximise utility, the vessel should be selected and prepared, and the site chosen, with due consideration to the local environment.

Trimix is a breathing gas consisting of oxygen, helium, and nitrogen. It is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

The helium is included as a substitute for some of the nitrogen, to reduce the narcotic effect of the breathing gas at depth and to reduce the work of breathing. With a mixture of three gases it is possible to create mixes suitable for different depths or purposes by adjusting the proportions of each gas. Oxygen content can be optimised for the depth to limit the risk of toxicity, and the inert component balanced between nitrogen (which is cheap but narcotic) and helium (which is not narcotic and reduces work of breathing, but is more expensive and can increase heat loss).