I have used Sikaflex 295UV, most boat stores carried it, however, make sure you are neat since that stuff is almost impossible to clean. It is a permanent solution and cost you less than $2o

 

X1 Sikaflex make excellent mastics/sealants and the marine versions are some of the best on the market. Sikaflex is used extensively in offshore oil platforms and has been for some 40-50 years - no better testing ground for quality and longevity in hostile environments.