The covered port

A USB socket, usually micro-USB on older stock and USB-C on anything current, with a rubber or silicone flap over it.

The flap works by compression: a moulded plug pushes into the socket opening and the material's own springiness keeps water out. That is a good seal when it is new. It degrades in three specific ways, and all three are common.

The tether tears. The flap is attached by a thin bridge of the same soft material, and that bridge flexes every time you charge. It is the highest-stress feature on the product. Once it goes, the flap is a loose plug that lives in a drawer.

The seal loses compression. Silicone and TPE both take a permanent set over time, especially when they are warm. A flap that has been compressed shut for a year does not spring back the way it did, and the seal it makes on the hundredth closing is weaker than the one it made on the first.

It gets closed with something in the way. Lint, a bit of towel, dried lubricant. The flap looks shut. It is not.

None of this is visible from outside, which is why people discover it by finding water inside.

The advantages are equally real: a standard cable, available anywhere, for a couple of pounds. When the cable dies you buy another one. That is not true of the alternative.

Magnetic contacts

Two or four flat metal pads on the outside of a sealed body, and a cable with spring-loaded pins that locates itself onto them.

There is no opening at all. The body can be a continuous moulded shell with no flap, no hinge and no compression seal, which is why this design appears on nearly everything that claims to be fully submersible. From a waterproofing standpoint it is simply better, and it is not close.

The costs are different and they are not trivial.

The cable is proprietary. Two pins at a specific spacing, sometimes with a magnet ring in a specific orientation. Lose it and the device is a paperweight unless the brand sells replacements, which many do not for long. This is the single largest source of otherwise-working devices becoming waste in this category.

The contacts corrode. They are exposed metal sitting in the one environment where salt, moisture and residue are all present. A film builds up, resistance rises, and charging becomes unreliable in exactly the way described in the runtime guide. It is cleanable, but it has to be cleaned.

Cheap versions get the polarity wrong. The magnet is supposed to make wrong-way-round physically impossible. On inexpensive implementations the magnet is weak or the pins are symmetric, so the cable can attach the wrong way. Some devices protect against it. Some do not.

The magnet attracts things. Steel wool is not a household item, but a drawer with hairpins, a key and a coin is. Small metal objects bridging the pads is a short circuit waiting for a charge.

How to tell which one you are looking at

Read the listing for two things: how many pads, and whether replacement cables are sold.

Two pads is power only, no communication, and no protection circuitry in the cable. Four pads usually means the device can talk to the charger, which is how better implementations detect the right orientation and stop at full.

If the listing says magnetic and does not mention a replacement cable, search the brand's own shop for one before you buy. If they do not sell it, you are buying a device with a built-in expiry date.

USB-C is the better port, when there is a port

If you are choosing between a covered micro-USB and a covered USB-C, take the USB-C. The connector is symmetric, so there is no orientation to get wrong, and it is rated for a far higher number of insertion cycles. More importantly, it is the cable you already own four of.

The sealing problem does not go away. The flap is still the weak point. But a USB-C flap on a device from a brand that has been making them for a few years is a better bet than a micro-USB flap, because the connector itself is less likely to be the thing that fails.

What actually breaks, in order

1. The flap tether on a covered port. 2. The cable on a magnetic design, usually by being left in a hotel room. 3. The contacts, through corrosion and residue. 4. The socket itself, through a bent pin or a connector pushed in at an angle.

The motor, the switch and the shell are all more reliable than any of those.