At sea, calling for help should never depend on mobile phone coverage. In this group you will find VHF radios for staying in touch with other boats and the coastguard. You will also find PLB beacons that raise the alarm via satellite.
A VHF radio is the standard means of communication at sea. It lets you hear weather and safety broadcasts, stay in touch with other boats and, in an emergency, contact the coastguard and nearby vessels directly. A handheld radio fits in a pocket, while a fixed radio gives you a longer range. Using a marine radio usually requires a licence and an operator certificate, so check the requirements where you fish.
A PLB, or personal locator beacon, is a personal device that sends a distress signal and your position via satellite to the rescue authorities. It is meant for situations where other means of communication fail, for example when an angler has gone into the water.
Many sea anglers carry both. Attach the PLB to your life jacket or clothing, register the beacon as instructed and check the battery before the season starts, so it is ready when it matters most.