In Abzu there are 19 hidden shells. 3 spaced throughout the primary environments and 1 at the very end within plain site. A lot of these shells are pretty easy to stumble across, but some of them can be tricky. With that in mind (and the sweet reward you get for collecting them all) I've scrapped this together.
Word to the wise, make ample use of the boost in this game. For starters, it makes swimming and breaching much more enjoyable. Secondly, and importantly, it will help speed up your collection.
(Be advised, I'm using a controller, which I think makes the game much more fluid - har har) So, the easiest way is to tap simply tap 'A'. This will give you a short term boost that doesn't really do much. If you hit 'A' again too soon, your diver will slow down.
For a large boost, hit 'A' and then immeaditely hit 'B' to perform a roll. This will cause your fins to glow gold and you'll have an incredible burst of speed when you exit your roll. This method, while quick, is not very easy to steer with.
The best method, I've found, is to time your boosts according to a rhythm. Initiate the first little boost and if you hit 'A' again you'll notice a larger boost. This will be accompanied by a much more pronounced dolphin swim. After this, time another 'A' hit just right and your fins will glow gold, giving you a huge boost. For an added kick, roll just as your fins turn gold. It will prolong the boost a bit.
Give it a try and get used to the rhythm. You'll get used to soon enough, and you can even string full boost chains together to continue swimming at peak speeds.