Holland airbase may sometimes be built on round 3 to deter an American airstrike on the German transport fleet after sealion. If you are seriously committed to sealion on the very first turn no matter what, then building the airbase G1 it is a better deal than a carrier because you will go into G3 with a battleship instead of a carrier. Normandy naval base will repair it for you so 2 hits to the battleship saves 2 planes just like the carrier, but the BB also attacks @4 and the base costs $1 less. The best part is they might take the bait and attack it and lose planes, which is good for Germany in any sealion scenario. More likely your extreme level of commitment to sealion should lead the allies to play conservatively with their air forces, so there will be no attack on your battleship, no Taranto attack, and USA may build in preparation for sealion rather than focus on Pacific defense. The big downside to it is you are basically locked into sealion because if you don’t go through with it, you will have to tie up an awful lot of resources defending it for minimal benefit.