Electrical – powering led strip from 2 alternating power sources

Increasing the voltage by that much will significantly increase the LED current - much more than you might think.

Most of those strips have 3- LEDs in series per section. The forward voltage on a Green or Blue LED is about 3.2V. The Red LEDs are about 1.7V each.

The intended voltage for those strips is about 12V. Let's use that as a starting point.

The increase in current for the Green and Blue LEDs is going to be about:

@ 12V: 12V - (3 * 3.2V) = 2.4V across the current limit resistors

@ 14V: 14V - (3 * 3.2V) = 4.4V across the current limit resistors

@ 16V: 16V - (3 * 3.2V) = 6.4V across the current limit resistors

So: increasing the supply voltage from 12V to 14V is going to increase the LED current by (4.4 / 2.4 * 100%) = 183%

Increasing the supply voltage from 12V to 16V is going to increase the LED current by (6.4 / 2.4 * 100%) = 267%

The situation is a little better with the Red LEDs. Doing the same math as above results in the following voltages across the current limit resistors for the Red LEDs:

12V: 6.9V across the current limit resistor

14V: 8.9V across the resistor

16V: 10.9V across the resistor.

That results in a 129% increase in current if running at 14V; a 158% increase in current if running at 16V.

I strongly suspect that the current limit resistors are going to be really unhappy. You will most likely also notice that the Red LED didn't get as bright as the Green & Blue LEDs at the higher voltages.

Bottom line: run the strips at the manufacturer's maximum voltage and feed power from both ends if possible.

IP68 indicates immersion resistance to at least 1m (3.2 Feet) or more, as described by the manufacturer. It should be just fine in a pool. Use adhesive heatshrink on the wire connections and coat the ends.

5V is not dangerous at all. You could put the leads in your mouth and not have any ill effects (except maybe over a week or two...). However, you should definitely have a fuse that will blow if excessive current is drawn. You can buy fuses and fuse holders on amazon. You should use a 2A fuse and a slightly-more-than-2A power supply, if you have one available. Your supply should have enough current to blow the fuse, or you can end up with the supply limiting instead of the fuse.