Once you reach Prov. Ivy you realize she has found a GS Ball and this is the item you were sent to retrieve. But before you are able to grab it, Team Rocket steps in and steals it.
Prof. Ivy mentions something about them having abnormal Pokemon and sends you toward Mikan Island to rescue the GS Ball. Surfing off the Eastern beach of Valencia, you are able to reach Mikan Island, but you cannot progress until you defeat the first Orange League Gym Leader, Cissy.
Cissy is a Flying and Water Type Gym Leader and, like all Orange Crew members, fights in a double battle. All gym leaders fight with 6 Pokemon 2 on 2 in an all out brawl. Defeating them will not be easy even after beating the Elite 4.
After Cissy you are sent to Navel Island, where Danny trains. He is Ice and Bug. but there are several other islands you have access to after Mikan. The ocean is free range, you can go anywhere your little surfing Pokemon can take you.