Finding you something with water views at the 1/2 million mark wont be a problem but to tell you what are the best cities in Florida, that will be hard to determine.
Your question states "Looking to relocate to Florida", therefore, you are opened to anywhere in Florida. People usually settle in an area that they think is the best one for them, Florida is mostly populated everywhere; so, people who live in Panama city (NW Florida) think it is the best place to live in FL, the same goes to all city residents, wherever they live is the best city.
The west coast of Florida on the Gulf of Mexico (from the Panhandle to Marco Island) offers the most beautiful white and wide sandy beaches. However, the cities are quainter and more suited for retirees who want a quiet life or families who are more homestead people. The northeast coast of Florida down to West Palm Beach also is for a quiet retiree of familial residential living but the beaches are narrower, though. From West Palm Beach down to Miami, the population is much more dense, (driving on US1 from West Palm Beach down to South Miami is like one big city (you need to read the street sign to know that you are leaving a city and entering another) but in that corner of Florida is where all the actions take place. It's like living in Manhattan with everything energized by the sun and the beaches. Southeast Florida is for people who are active and energetic and on the go all the time. However, the Tampa Bay area (mid west coast of Florida) and Orlando (mid and center of Florida) are huge energetic cities, also.
You need to decide what kind of lifestyle you want. When you have narrowed your location within the state of Florida, then you can inquire about cities and their neighborhoods based on your $500K purchasing price. And William Hundridge (below) is right, $500K won't buy you directly on the ocean anywhere in Florida but it will get you a house on water (canals) near the ocean.
I hope that helps you. If you chose to settle in SE Florida, call me, anytime.