We've been in those forward-facing penthouses on the Gem and Jewel several times. Yes, I believe there's a sign about not being out on the balcony when the ship is underway. But, in practice, there's no issue being out there when the ship is at sea; in fact, the folks on the Navigation Bridge can see you out there. Never been a problem.
Just be careful to have the sea door open and hooked to the bulkhead behind it. And *never* open your stateroom door when the sea door is open and the ship is moving. It's a mistake you'll only make once. đ [And don't let your fingers anywhere near the frame of the sea door when you are opening or closing it.]
One of the most relaxing days I've ever spent was sitting out on that balcony during the sea day after we'd transited the Panama Canal, watching a bunch of Brown Booby birds flying around and landing on the ship, knocking each other off, etc.