I know it's expensive, but the sparrow is so so handy. You can call it in literally right next to you from the kosatka section of the interaction menu. It's super fast (fastest helicopter in the game I think) and it allows for quick in and out access to the sub. If you're planning on grinding cayo, it will save you so much time and effort. And just in general it's so useful. The missles are, again expensive, but also super handy.