沖縄おきなわに行いくとき、船ふねで行いきました。
When I was going to Okinawa, I went by ship.

S1（V-る／ V-ている／ V-た）とき、S2: Used when expressing when some event or action occurred.

Structure

来月らいげつ沖縄おきなわに行いくとき、ひこうきで行いきます。
When I go to Okinawa next month, I will go by airplane.
ダイビングをするとき、どうぐをかりました。
When I went diving, I borrowed the equipment.
バスに乗のっているとき、ガイドさんのせつめいを聞ききました。
When I was in the bus, I listened to the bus guide's description.
ふねはゆれました。ついたとき、ほっとしました。
The ship swayed. When we arrived, I was relieved.

“S1 とき” indicates the time when the action in S2 occurred.
When using the verb dictionary form, the actions in S1 and S2 are occurring simultaneously.
The dictionary form is also used when the action in S2 is done before the action in S1.
When using the “～ている” form of verb, S2 occurs while doing the action in S1.
When using the “V-たform” of verb, the action in S2 occurred after the action in S1.
The tense is expressed by S2.