Here is some advice that will probably never come in handy. If you ever find yourself playing hide and seek in the deep sea, your best bet is to find a barrel sponge (Demospongiae). These huge beasts of barrel can reach up to 2 metres in size, far large enough to conceal a person and stump the seeking player.
However, large specimens might be considered too old for a juvenile game of hide and sponge. As they only grow 1.5cm a year, you can wager that any useful hiding sponges are over 100 years old. They can be located at a depth range of 10-120m and come in a variety of colours, becoming paler as depth increases. Their brown, red-brown or rose-purple colouration is, in part, due to the symbiotic algae within the sponges’ tissues.
Unfortunately, you’d need a time-machine to see the largest known barrel sponge. This Barrel-rog played a bit too much hide and sponge. In its prime back in the early 90s, it was 2.5m in diameter, making it quite a celebrity in Curacao (Carribean). Sadly, fame has its price and the sheer number of admirers touching the sponge infected its tissues. By 1997, it was a mere shadow of the healthy giant that it was back in the day.
On second thoughts, maybe think twice about including barrel sponges in deep water hide and seek!