Contrary to most beliefs, you don't need an expensive GPS for kayaking as some of the features are only correct if the GPS is always kept on and in kayaking, that is not the case. If you are only paddling for GPS a day trip, you might keep the GPS on while continuously paddling, but in most situations, you are concerned about the life of the batteries. A GPS never replaces good navigational skills and never replaces a real map. You must always carry a map and know how to read the map. A GPS doesn't know about all the outside factors that might be occurring: tide changes, rough seas, changing weather conditions, etc. It is the knowledge of all factors, along with the use of a GPS, that makes your trip safe.
Read about "How To Use A GPS While Kayaking"