What type of lens should i use to shoot a beach wedding?

I am a learning photographer and I have been taking a lot of family pictures. My husbands co-worker needs me to take their wedding pictures for them as they can not afford a photographer. I have cannon rebel and 18-55 and 75-300 lens. I have a diffusion filter along with the UV filter as well. I really want to to the best job I can I enjoy blur backgrounds as does the bride which lens do I need to get or can I use along with do I need another filter. Also the wedding is at 5pm at VA beach in May it will be a very quick wedding and I may not have much time to change lens and settings. I usually shoot in no flash because like natural lighting. Do I need to buy a additional flash? Also should I use a tripod? Please help as this is my first wedding of many I hope. I am doing this for free and I want to do the best job I can.