The external pressure that is being in this problem is the osmotic pressure produced by the seawater.
The formula for the osmotic pressure is
P = CRT
where
P is the osmotic pressure in atm
C is the concentration in moles/L or M
R is the universal gas constant = 0.0821 atm-L / mol-K
T is the temperature of the solution (assume 25 C = 298 K)
P = 1.14 (0.0821) (298)
P = 27.89 atm