The VLF (very low frequency) band, which is 3kHz to 30kHz, is able to penetrate water a couple of tens of meters. That makes this band best suited to communicate with a submerged submarine.

The military satellites use a blue/green or aquamarine color LASER beam to communicate with submerged submarines.

Modern submarines are designed to go much faster submerged. This design also limits their surface speed capability.

WW1 and WW2 submarines were not considered true submersibles by today's standards. They were essentially surface vessels that had the capability of limited submerged operations. As such, their design and propulsion made them faster on the surface and much slower underwater. Modern submarines are true submersibles in that their only limitation is that of their crews.

The primary method of detection of submerged submarines is sonar. We also see magnetic anomaly detectors used at close range.

To withstand high water pressure when they are submerged at sea.