You can go anywhere you went before, however you should probably head east of Mauville, via surf and then head north as then you will come across the weather institute and eventually the city of Fortree where the 6th gym is. Other than that, yeah you cant do much.
Beat the Gym Leader
You get it after you beat the elite 4.
beat the grunts
On Pokemon ruby to get surf you need to beat your dads gym (in petalburg) then after wallys dad will come and get you then give you surf.
most water Pokemon
your starter pokemon can be a water type and you can trade water type pokemon with your friends in pokemon ruby
water or fire type on a high level (above his)
you only have to beat the elite 4 on Pokemon fire red not ruby
just surf on the ocean..
you play it
Use strong water or dark type Pokemon
A fighting type, a ghost type, a rock type, a ice type, a water type.
The Pokemon League
I'm going to assume you mean "Pokemon Ruby" in your question. Anyway, to trade between Ruby and XD, you must beat Pokemon XD and defeat the champion in Ruby.
8th gym leader
No, but you must get to a certain point in Pokemon Diamond to transfer Pokemon from Pokemon Ruby.
well what you need is a:pyhsic,dark or a fighting type and get them to a good level like 47 or somthin and you will beat them!! GO WATER POKEMON!!