There seem to be two Sea Air websites:
SEA AIR TOURIST AND TRAVEL LIMITED - Günstige Flüge nach Deutschland, Kroatien und Spanien - PADERBORN, ALMERIA, PULA, , IBIZA, PALMA DE MALLORCA
Home | Sea Air
Only the first website makes mention of a Southend-Munich route.
Looking at the first website, I found the address of Sea Air - it comes up as 83 Ducie Street, Manchester, M1 2JQ
SEA AIR TOURIST AND TRAVEL LIMITED - Kontakt mit SEA AIR TOURIST AND TRAVEL LIMITED
I don't live in Manchester, so had a quick look at Google Maps to see whether the photos of the address seems plausible for a travel agent - it does not. It looks like a seriously run down street, with heavy metal shutters on many of the windows to protect against vandalism.
I had a look at Royal Mail - the address seems to be possibly some sort of forwarding / brass plate address / mailbox rental controlled by City Address Ltd
I then had a look at Companies House - I noticed that Sea Air Tourist and Travel was incorporated on 14-Jan this year - i.e. barely two months ago. Needless to say, there are no filed accounts. From 14-Jan-2016 to 04-Feb-2016 the company was based at 152 London Road, Morden, London which according to Google Maps appears to be a residential address.
The company seems to have share capital of 1 pound (essentially the legal minimum), and one director called 'Nando Kos' who seems to appear on various websites as being linked to Sea Air.
Both website
www.sea-air.info and
www.sea-air.hr are registered to a Nando Kos with the given address being located at Osijek airport. sea-air.info seems to have been possibly registered on 27-Feb-2016
I have found nothing whch says this is anything other than entirely legitimate, but I'm keeping my credit card in my pocket until I see a Sea Air branded flight land and take off again at Southend.