It's impossible to guarantee a coast start. When to start bias is set to coastal the game will try to place the player along the coast. But it will only happen if the game code can find a valid starting location along the coast.

If I remember correctly you can use the region avoid setting to avoid dessert and tundra. But again the result will not be guaranteed.

You can also use this mod to fill in the terrain around your starting spot with terrain other than dessert or tundra. But that has the possibility of causing some funny looking maps.