For silicone dolls, typically no, water cannot get trapped inside the doll as the dolls are solid silicone. The exception is if the doll has the standing foot bolts or an unseen unnatural opening somewhere in the body that leads to the skeleton The bodies of the dolls are not hollow.
For TPE dolls, yes, it absolutely can. TPE dolls are porous and they absorb. Also, many TPE dolls come with the standing foot bolts and do not have the seamless heads like silicone dolls. This means there is a head and a neck bolt, in which water can enter and become trapped. TPE is also more prone to tears and damage than silicone, so again, there can be unseen unnatural openings, specifically in the anal/vaginal/oral cavities that can allow water to enter.
Click here for a link to a doll which had water trapped inside. * WARNING * Images may be uncomfortable for viewing, as it shows doll surgery.
For both silicone and TPE dolls, after bathing, it is imperative that you make sure your doll is thoroughly dry. The use of tampons or an aquarium pump with hose to dry out the cavities is a most excellent idea.
For TPE skin, we recommend that you thoroughly dry your doll and sprinkle baby-powder or corn starch on the skin to keep it moisture-free and dry.