The Sea Based Walrus has to place. It can't stay "on the ground" or fly to the land airbase. This was clarified with the Rufe in the official Rules Clarifications:
"A6M2-N "Rufe": These units must observe stacking limits when returning to base (four aircraft squadrons per sector). If there are no coast or island sectors on your side of the map, then this unit uses any sector in your Ship Deployment Area as its base. Rufes must be placed during the Air Mission Phase; they cannot be sent to the land airbase or remain at their base (but can be placed "in the air" above an island base)."