Isn't the end. The novels have sequels. Something about a trip to the sea and mermaids, festival, reopening of the gate, JMSDF submarine cruising the seas... Look it up.
Book Walker (English language site) has region blocked only Japan. Maybe there are other restrictions I don't know, such as your home country limiting availability of credit cards or where to pay with them. At least something you can check out if you haven't already.