No. These lines, often reproduced in atlases are simply to show which islands belong to which political units. They are not boundaries with any legal standing. They use straight lines just because any curve (say a circle around Nauru) would mean concavities for another political unit and thus appear unfair/uneven/confusing.
Maritime political boundaries which are drawn up between two (or more) countries to delimit their respective maritime zones (is territorial waters, EEZs, continental shelves, so water or seabed) are usually equidistant lines between islands in each political group.
Surprisingly few have been actually drawn yet, usually because countries in the Pacific have better ways to spend their time/$ than drawing lines at sea, or because there are disputes over ownership of islands or whether low elevations such as reefs can count as points from which to measure an equidistance line.
So for example, the Vanuatu/New Caledonia(ie France) dispute over who owns Matthew and Hunter islands prevents those two countries drawing up their maritime boundary.
Samoa, which is hemmed in by other island groups, hopes to get concessions from its neighbours to ensure it has a reasonable EEZ compared to them, given its large land area and population compared to the neighbours.

Brendan Whyte

> Date: Sat, 06 Mar 2004 21:34:25 -0000
> From: "Artur Kroc" <kroc@...>
> Subject: Re: caprivi strip - Yes Zimbabwe touches Caprivi
>
> --- In BoundaryPoint@yahoogroups.com, Ernst Stavro Blofeld
> <blofeld_es@y...> wrote:
> >
> > --- "L. A. Nadybal" <lnadybal@c...> wrote:
> . This is, to my knowledge, the only point in
> > > the world
> > > (quadruple point) where four countries meet at one
> > > point...
> Only one? I think, that another one is maritime quadripoint: Nauru,
> Solomon Islands, Tuvalu and Kiribati.
> http://go.hrw.com/atlas/norm_map/oceania.gif
> http://www.worldstadiums.com/oceania/maps/oceania.shtml
> Is it true?
>