In Pokemon red and blue version, go to the department store in celadon city, buy a drink from a vending machine, and give it to one of the guards around saffron city. They will share the drink with the other guards, and all will give you access.
In fire red and leaf green, you follow a similar procedure, but instead of getting a drink from the vending machine, talk to the lady on the first floor of the Pokemon mansion (also in celadon city), and she will give you tea for you to give to the guards, instead.
You can teach your Pokemon surf and go surfing to run into a water type option. Or fishing which I prefer because if you are using the right rod you can get some great Pokemon.
either trade with someone or use surf to find some or go in the grass that is next to any water
you go near rock tunnel and then you will see some water then just surf on the water
you need a water Pokemon
get a hm called dive attach it to a water type pokemon and go on water and use dive.
go to the city with the big store and go into the tan skyscraper and talk to the ld woman
you have to teach it to a water Pokemon and then go to Pokemon, select the Pokemon you taught it to and you can dive.
no
you go to pastoria city and go to the water and go ing a house by the water.
for black and white you go to tubeline br and pause then press abayxbay
in Pokemon platinum if you have a water type Pokemon go to the tutor men in pastoria city
go fishing mate :)