I think that brawling should only take away 5 atk power, because you can only attack once, but hundreds of players can attack you once per 6 hours, so it gives us a 1:100 Ratio (Over-exaggerated numbers) but I hope you get the point. But the system where, you can only equip pirates to your squad based on your atk/def power, that should stay the same, only, that when you brawl, you only lose 5 atk points, rather than the points that your crew is worth.
I disagree XD With everyone having to only lose 5 ATK Points during a battle will only increase your problem being attacked, ironically enough. Reason for this is because players will have a lot of ATK Points to spare; meaning that they will be able to brawl more variety of players in a short period of time (Which will potentially raises your chance of getting attacked constantly by other players 24/7, perhaps to the point that the 1:100 ratio won't be an exaggeration! XD). (´･ω･`)
Also, high level players with a monstrous ATK Crew will go Brawl-Happy knowing that they will only lose 5 ATK points in a fight, so this mechanic can be disrupting to game balance (Not to mention chaotic as well). (´･ω･`)
As ATK Points replenish at a rate of 1 every minute, it shouldn't take you six hours to be battle-ready again. If you're constantly getting attacked, then some group may have a grudge on you or your DEF is noticeable to easily defeat (So perhaps invest in DEF Points?).
Suggestion: - A brawl history of those you have attacked.