Hi
Kitty,
Welcome to the forums. What you will have to do is create a selection around the person who's image you wish to place above the sea and then extract the selected portion of the image and place it on the new background. There are a lot of issues which we would keep in mind to merge two images...light source, shadows etc.
To begin, I hope you have a big enough image to start with. In helping to organize your approach to this project we need some basic information. Open the image in Photoshop (what version of PS will you be using?) and use Image > Image Size... to determine the size of the images you are starting with. Tell us the pixel sizes of the images with which you will be working. Then, what size are the posters...your final output? Will they be printed commercially? What resolution does the printer require?
In order to create a poster in PS we have to know the output. Now about extracting the person from one image to float them above a sea, we really need to see the image to give you the best advice. There are many techniques in Photoshop to create selections, each of which may be applied to best advantage in different circumstances.
To show us a copy of your image, you will have to save a copy as a jpeg no bigger than 600px wide and upload it as an attachment. So open your image in PS and then use Image > Image Size... to resize the image to 600px wide. Then use Save For Web and save it as a JPEG no bigger than 100K. Once you have that you can close the image without saving and the original won't have been changed at all.
Do understand that you can do that project, with help, even completely new to Photoshop, but it will be a challenge to you and us to help. We'll try!