In order to make traversing through water a little easier, your brother may want to disable the "Water Collision" option. Water Collision keeps the camera above water when the character is above the surface and below water when the character is beneath the surface; so it may make it difficult to dive down under the water (which, by the way, can be accomplished by right-clicking and moving the mouse so that the camera is looking downwards).
To disable the option, have your brother open up his Main Menu (ESC) and enter Interface Options. Once there, suggest that he click on the Advanced Options tab, and then ensure that the Water Collision option is disabled. Once he presses Okay to save his changes, your brother should be all set. I hope this helps, Massedar. :)
Coreiel broke 25% of my heart. </3