You get it on Route 20, east of Cinnabar Island in the water, you will need the Dowsing MCHN which you get from Ecruteak City by talking to a trainer inside of a house.
route 20 next to chinabar and deepseascale other side of rocks by seaform island
You either need a DeepSeaScale or DeepSeaTooth
The only Pokemon that evolves with the deepseatooth is Clamperl, who, using the deepseatooth, evolves into Huntail.
clamperl
Clamperl when traded
Trade it holding the DeepSeaTooth or the DeepSeaScale. DeepSeaTooth will give you Huntail, DeepSeaScale will give you Gorebyss.
The deepseaclaw is not an item there is a deepseascale and deepseatooth.
In Pokemon emerald, ruby or sapphire: Get a Camperl and a Deepseatooth, equip it to Camperl, and trade it. It will instantly evolve into Huntail. Be careful, if you've already chosen the Deepseascale, you cannot get a Deepseatooth.
FIrst you must get a scanner. Then you give it to a guy in the slateport port. He will let you choose between deepseatooth and deepseascale.
Either in Route 13 or Route 17.
If given to clampearl then traded it will become huntail.
Clamperl evolves by holding a DeepSeaScale or a DeepSeaTooth and then trading it to another game, a DeepSeaTooth evolves it into a Huntail and a DeepSeaScale evolves it into a Gorebyss.