Could I build an island in the Pacific ocean and claim the throne?
<p>If I construct an island in the middle of the Pacific ocean (not within any country's water territories) could I claim sovereignty?</p>

<p>This is covered by the <a href="https://en.wikipedia.org/wiki/United_Nations_Convention_on_the_Law_of_the_Sea" rel="nofollow noreferrer">UN Convention on the Law of the Sea</a>.  You specify that the island is constructed "not within any country's water territories": this is the "high seas", covered by <a href="https://www.un.org/Depts/los/convention_agreements/texts/unclos/part7.htm" rel="nofollow noreferrer">Part VII of the treaty</a>.  Specifically relevant to you is Article 89:</p>

<blockquote>
  <p>No State may validly purport to subject any part of the high seas to its sovereignty.</p>
</blockquote>

<p>Yes, you can construct an artificial island (Article 87, part 1d), but you can't claim sovereignty over it.</p>

international sovereignty