You have the context to judge by, but given your reference to "description of a boat" and the following quote, it appears to me that we are talking about the superstructure of a boat, not the quality of construction.

<<Certificat de jaugeage établi à : Les Sables d'Olonne : 6/12/1955.
Navire en : Bois.
Nombre de mâts : 2 mâts.
Nombre de ponts : un pont.
Longueur totale : 16,37 mètres
Plus grande largesse extérieure : 4,82 mètres.
Hauteur au milieu du navire (creux) : 2,00 mètres.
Volume de la coque : 26,52 tonneaux.
***Constructions supérieures*** : 1,09 tonneaux.
Jauge brute : 27,61 tonneaux.
Jauge net : 6,00 tonneaux. >>

I'm not a boatie, but I associate "superstructure" more readily with a liner, freighter, warship, etc. than with a small yacht, say.

That said, the following definitions of "topsides" appear to be slightly in contradiction with the above quote (which distinguishes between "volume de coque" and "constructions supérieures", the total being the "jauge brute", whereas a/c to what follows, topsides encompasses the part of the hull above the waterline.

<<TOPSIDES - The sides of a vessel between the waterline and the deck; sometimes referring to onto or above the deck.>>

<<topsides
The hull above the waterline. Also, everything above deck as opposed to below deck. >>



--------------------------------------------------
Note added at 2003-11-04 21:01:25 (GMT)
--------------------------------------------------

Maybe \"deck structures\" would be more specific.