Swimsuits / Trunks
Swimsuits come in all shapes and sizes. If it is comfortable then it is good.
Men should look at briefs, trunks or jammers to avoid extra drag.
If you are looking to increase your speed through the water you do not need to spend a fortune on a racing suit, unless you are already at a competitive / elite level.
Goggles
There is no perfect goggle as they are a very personalised piece of equipment based on the shape of your face. Try a few brands until you find something that is comfortable and has a good field of vision. Fully customised goggles are available but they are much more expensive.
Wetsuits
If using a wetsuit it is critical that you use a swimming specific one. These have much more flexible shoulder material that allows for the movements required in swimming. Most will be designed with freestyle in mind. The more established brands cater for different swimming ability, which provide more or less buoyancy as required.
Swimming Caps
The primary purpose of a swimming cap in open water swimming is to make the swimmer more visible to other water users.
Generally they come as latex, silicone or neoprene. Silicone caps are easier to use than latex ones and are more durable. They are also the most hydrodynamic and can by doubled up if necessary. They are available as racing and larger volume caps. Neoprene caps provide more insulation and can cover the neck and chin as a hood.
Tow Float
Tow floats are used in conjunction with swim caps to make the swimmer more visible to other water users. They come in a variety of styles and can also be used to carry keys, phone and clothes. Some tow floats are specifically designed to carry a larger load and are used for adventure swimming.
Paddles, Fins, Kick and Pull Buoys
Paddles and fins are used to improved technique and increase swimming specific strength.
Kick and Pull buoys are used to isolate certain aspect of your swimming stroke, allowing you to focus on a certain techniques. Kick buoys remove arm movement so you can focus on kicking, pull buoys do the opposite along with countless other drills.