For your issue with Adventure and spawn camping, you can Scuttle your Ship. If they didn't repair and keep you up, the end result would be you sinking. So accept that fact and Scuttle, they will not be able to save the Ship anymore, you get a new Ship somewhere away from them. Problem solved.
As it pertains to Arena, what you describe is a risky strategy. It might work out for them, it might not work out for them. Depends on the match, depends on how the ambush plays out, it can all go south for them very quickly. That doesn't sound like something that needs a change to me, though I'm sorry you got taken down by it.
The reality is, you're going to lose encounters with other players in this game. You'll win some too, which is great, but you need to be able to accept that you're also going to lose. Sometimes you're even going to lose out big time, loads of loot and hours of trying to get it.
You can do things to mitigate these loses, and you can also wrap your head around the idea that none of it matters beyond getting on the game and having some fun. There will always be more Gold, Doubloons, Silver, and Ancient Coins to get your hands on. Some of the greatest legends have many moments of failure and loss, but they are legends because their story doesn't stop there. Pretty well the same context in the game.
I wish you luck on the Seas, if you feel like you could use tips and advice to up your chances, the community will have loads to share with you.
How to Scuttle Your Ship in Sea of Thieves
