Sea Navies are there to help monitor for rebel spaceship emergency landings, but also for non-rebel spaceships requiring a Splashdown landing
One thing about other countries on the planet that would likely cause an issue is landmass - if there's enough landspace for interplanetary travel via spaceships, which country houses which landing pads would be a contentious issue - since that could be farmland, or residential building lands, or mountains, or...
Thing is, other planets could be contacting via spaceships, and the landing pads sort of likely need to exist for communication, and supplies, or the transport of people between planets.
Which means you'll have a lot of details around immigration and exports/imports in the country, and you'll want to cut down on rebel smuggling attempts and such. If an unauthorized spaceship attempts entry, they could choose not to even land at your landing ports, and just land in any ocean, or on a deserted island, and stay hidden. 199
Not great for a world government. But since you'll have to check the oceans and seas anyways, that seems like a great place to put your official landing ports for official, authorized travel.
So, what you'll want to do is have your sea navy in charge of air traffic control essentially for interplanetary travel - when you attempt to land, the following happens:
1.) You contact the local sea navy by planetary comms from outside atmosphere asking for an entry point, with maybe some guidance on which continent or city you want to depart at;
2.) The sea navy gives you coordinates to land at sea near the departure point, and send their landing pad out to that coordinate.
3.) You then enter the atmosphere, aiming for a targeted landing at that location.
4.) You hopefully* land on the landing pad at sea.
5.) You're transported by a ferry accompanied by the local navy ships and end up at your destination.
6.) Your ship is docked such that, for takeoff, you can take a landing pad out to see and launch off of that once you're ready to leave.
*This keeps the country from having to worry about mishaps in landing mistakes - instead of setting fire to a landing pad on land, and having to worry about their forests as much, if you do cause a fire explosion with a mistaken landing, they can just dunk the landing pad or your ship into the water to reduce the effects. Ideally, this doesn't happen, but if it does, that can be contained safely.
Anyone attempting to land without these steps is probably a rebel, so you encounter them with the navy if they land in the sea - which is likely their easiest target anyways, because there's likely to be a lot more water that is safe to land at than land with cities, or farmland, or notorious parks. 500