Why is colder seawater more compressible?
First, let's explain what compressibility is. Compressibility is defined as a measure of the relative volume change of a parcel of fluid as a response to a pressure change. 

When I calculate the compressibility of seawater using Matlab and the seawater package (equivalent to Fofonoff &amp; Millard, 1983), I obtained that the compressibility increases at cold temperatures for salty, brackish, and fresh water. In fact, water has a minimum compressibility at 46.5°C (Fine &amp; Millero, 1973). As pressure increases the compressibility decreases for any of the salinities considered. 


As can be seen in the figure the compressibility is maximum at low temperatures for any salinity (not just seawater) and pressure. So in general, the compressibility of water is maximum at low temperatures due to the internal structure of water. The inverse relationship with temperature seems well accepted (Fine &amp; Millero, 1973) and has a direct relation with other properties like the speed of sound.

In a typical liquid, the compressibility at low temperatures is reduced as the structure becomes more compact. In water, there is a change at low temperatures toward more open-structure clusters (expanded icosahedral water cluster) versus denser clusters (collapsed icosahedral water cluster). As the water structure at lower temperatures is more open, the capacity for it to be compressed increases. 

As cold liquid water is heated it shrinks, it becomes less easy to compress, the speed of sound within it increases, gases become less soluble and it is easier to heat.

The compressibility above is calculated using the formula in the figure (same as in the wiki article) and I have included the code to reproduce the results:

T=-2:1:30;
P=0:10:1000;
S=35;
for i=1:length(T)
    for j=1:length(P)
        dens(i,j)=sw_dens(S,T(i),P(j));
        dens15(i,j)=sw_dens(15,T(i),P(j));
        dens0(i,j)=sw_dens(0,T(i),P(j));
    end
end
spvo=1./dens;
spvo15=1./dens15;
spvo0=1./dens0;    
for i=1:length(T)
    for j=1:length(P)-1
%        Ev2(i,j)=-(1./spvo(i,j))*(spvo(i,j+1)-spvo(i,j))./(P(j+1)-P(j));
        Ev35(i,j)=(1-spvo(i,j+1)/spvo(i,j))/(P(j+1)-P(j));
        Ev15(i,j)=(1-spvo15(i,j+1)/spvo15(i,j))/(P(j+1)-P(j));
        Ev0(i,j)=(1-spvo0(i,j+1)/spvo0(i,j))/(P(j+1)-P(j));
    end
end
figure(1);clf
for j=[1,20,100]
    line(T,Ev35(:,j),'Color','k','LineWidth',2);
    line(T,Ev15(:,j),'Color','b','LineWidth',2);
    line(T,Ev0(:,j),'Color','r','LineWidth',2);axtt
end
text(T(20),Ev35(20,1),['P=',num2str(P(2)),'dbar'])
text(T(20),Ev35(20,20)*.99,['P=',num2str(P(21)),'dbar'])
text(T(20),Ev35(20,100)*.98,['P=',num2str(P(101)),'dbar'])
text(T(15),Ev0(15,1),['P=',num2str(P(2)),'dbar'],'Color','r')
text(T(15),Ev0(15,20)*.99,['P=',num2str(P(21)),'dbar'],'Color','r')
text(T(15),Ev0(15,100)*.99,['P=',num2str(P(101)),'dbar'],'Color','r')
xlabel('Temperature (degC) ')
ylabel('Compressibility (1/dbar) ')
legend('35 PSU','15 PSU','0 PSU')    
text(7,5e-6,'$c_w=-{1\over V}({dV\over dp})$','Interpreter','latex','FontSize',20,'FontWeight','bold')