What you are asking about is similitude, the linked wikipedia article does a very good job of explaining what you should aim for. You are also going to want to be familiar with Buckingham's $\pi$ theorem...
Consider the impact force, $F$. It will clearly depend on some parameters of the model cpasule, say its size, $L$, its mass, $M$, and the velocity of impact against the water, $V$. It will also depend on some parameters of the fluid, like its density, $\rho$. We could also consider viscosity, $\mu$, but lets leave it out of the picture for now. So we have 5 variables $(F,L,M,V,\rho)$ that depend on 3 independent physical units (mass, length, time), so we can rewrite their dependency in terms of $5-3=2$ dimensionless quantities. A good option is to use a density ratio, $\alpha = \rho L^3 / M$, and a force ratio, $\beta = F L / MV^2$. So we know that these two quantities will obet some functional relation, $f(\alpha, \beta)=0$, which we can rewite as $\beta = f(\alpha)$, or more precisely for our needs as
$$F = \frac{MV^2}{L}f(\alpha)$$
If you keep alpha unchanged, i.e. scale length by $1/3$ and mass by $1/27$, in order for the force $F$ to be the same in the model and in the full scale capsule, you need to scale the velocity by $3$. This would, incidentally, also keep viscous effects unchanged.
Of course, because you are applying the same force to something 27 times lighter, accelerations are going to be very different. If you wanted to keep those the same, then you would want to see what happens with
$$\frac{F}{M} = \frac{V^2}{L}f(\alpha)$$
and you will now want to scale your velocity by $1/\sqrt{3}$ to get the same acceleration.
On the other hand, if the worry is about structural integrity, this is normally related to stress, so you could go with
$$\frac{F}{L^2} = \frac{MV^2}{L^3}f(\alpha)$$
and now, to keep stresses the same, you will want to keep velocity unchanged from that of the real capsule.
