Stand up paddleboarding is a very safe activity if you wear the correct equipment, use a good quality board and properly plan for weather conditions.
Particular attention should be paid to wind and tides. Stand up paddle boards are large and it is possible to be blown offshore, or caught in strong rip currents. A paddleboarding guide will always monitor these weather factors before any SUP adventure.
When you are in the sea your SUP is your personal flotation device. If it deflates then this could be very dangerous. Cheap poor quality SUP boards should be avoided.
You should always wear a wetsuit when on your SUP. If something untoward does happen then your wetsuit will help keep you warm, as well as providing flotation to you in the water.