I'd like to share some information here -- as for suppliers, there are many, but you may wish to choose one close to you, or one that might sell everything you need in one purchase -- it can take a little looking around at first.
For seascape gel candles, I've found that medium density ("MP") Versagel is the nicest one to work with for my designs. It comes in three densities -- the light one does not allow for suspension of embeds too well; the heavy density is trickier to work with in that some people have more issues with bubbles -- I don't mind the heavy myself, but if you're just starting out, the medium would be best. The scents used in gel candles need to be specific scents that are okay to use with candle gel.
You might start here: http://www.candlesandsupplies.net/Ca...ing/Candle-Gel
Then go to their fragrance oil section where it says "Candle Gel Fragrances."
Your wick type is going to be determined, in part, by the diameter of your container.