If you are actually swimming: a one-piece or a racerback. Strappy and cage cuts are built for how they look standing up. Diving into a pool in one is how people lose a bikini top.
If you are a different size top and bottom: buy separates. It is the single most useful thing about this category and the thing most people do not think to do. A set locks you into one size for both halves.
If it is mostly for photographs: metallic, hologram and sequin. Flat colors go dull in bright sun. These finishes are the reason the category looks the way it does.
If it is going in chlorine or salt: rinse it the same day. Both eat elastic, and metallic and hologram finishes dull faster than plain fabric does. Cold water, no machine, dry flat out of direct sun.
If you want it to work at a festival too: buy the cover-up at the same time. A bikini under a sheer dress or a mesh tank reads as an outfit. The same bikini on its own reads as swimwear, which is a different day out.
On lining: check it before you order a light color. Unlined pale fabric turns transparent when wet, which matters far more in a pool than it does in a dressing room.
