Re-bedding Windows On A 40ft Sailboat. Am I using the Wrong Sealants?

Todays boat work and renovation on our sailboat is rebedding the coachroof windows. What sealant should I use to do this job in the coming days? Help me out here guys and leave me a comment. Sikaflex 291i, Fischer Marine sealant, Builders Adhesive sealant or cheap Silicone Sealant? Is professional marine adhesive sealant a scam or the only way?