LED Strips input supply directional
Figure 1. A typical LED strip.



Figure 2. The typical power-rail and three series LEDs arrangement. Source: LEDnique.

The required current limiting resistance can be made of one or two series resistors.

It should be clear from the schematic that if the strip is cut as shown in Figure 1 that the lone LED on the left and the two orphans on the right will have lost their connection to the supply rails. The three sets of three in the centre will light as expected but the outer ones will not. This is the reason for the scissor symbols.




  I understand that Diagram A is the correct +12V Input supply and I can measure the end of the led with the same voltage ... but the end of the LED [B] is 0 V.


The important thing is that there is 12 V between the rails at both ends.


  I already installed the LED strip underwater with Diagram B. I want to know will my mistake break my PSU and can cause issue on both LED and PSU?


Provided they are insulated against water ingress there will be no problem. You can feed those strips from either end.