He said the more you train and play, the longer your body can cope under water.
The 16-year-olds have been playing for about four years now and have impressed the New Zealand selectors, earning a spot in the national under-18 team to play Australia in July.
Rotorua's Helen Payn, 18, who is currently attending university in Wellington, has also been chosen to represent the country.
She will play for the New Zealand under-18 women's team at the same tournament in Wellington.
"It felt really good to be selected, because I had achieved something that I had been aspiring to for about five years," Payn said.
"It first started with rowing, where I developed into wanting to represent New Zealand, and I have ended up representing New Zealand in underwater hockey."
The Rotorua trio all said they would like to perform well in July to put their hands up for selection again next year.
The world champs are being held next year in Spain, which would be a great opportunity for the talented locals.
Shields said the sport was quite addictive once you got started.
"It is hard to explain unless you have played it ..."
Underwater hockey is played at the bottom of a 2m-deep pool. Players wear a mask, snorkel, fins and a glove to protect them from the hard puck. Each player uses a short stick to control the puck under the water, similar to a hockey match. Each team has up to 10 players but only six can be in play at any given time.
Rotorua will host a couple of big underwater hockey events this year, including the New Zealand Secondary Schools nationals on September 30. Teams from throughout the country are expected to attend the event.
