Why does it have to be this

The trick to extending the life of your bathtub/shower area is to keep
water from penetrating through the finished surfaces of the walls and
floors.  For the most part, it is simply a matter of keeping an eye on
the situation, and tending to minor breaches as soon as they occur.

The most common tub area surfaces are usually tile of some sort or
plastic of some sort.  Either way, the area is a patchwork of junctions
between numerous pieces.  Every single one of these joints must be
sealed against water penetration.  For tiles, the sealant is grout.  For
all other joints, the sealant is caulk.

Cleanliness is better than holiness

Cleaning is the first line of defense.  If you permit wildlife to flourish in
your shower stall you can expect the milieu to heed the call of the
wild.  Mold and mildew, beside being unsightly, slowly degrade grouts
and painted surfaces.  Their microscopic “roots” burrow into unglazed
surfaces, compromising their water resistance.

Bleach based cleaners work well at removing these invaders.  I have
found that cleaners tend to work best if they are left to sit and “soak”
for 20 or 30 minutes after being sprayed on. Even though the
cleaners may appear to be dried up when you go back after that
length of time, they have done their job.  The dead vegetation easily
cleans off with minimal scouring by an abrasive sponge.

Grout: the adult’s answer to Playdough

As you clean, keep an eye pealed for any loose or missing grout.  
Even the smallest holes will eventually lead to noticeable water
damage.  Any faulty grout should be immediately patched with new

Patching small grout sections need not be a major production.  Use
some kind of pointy object (a kitchen knife, a carpenter’s knife, or
even a nail or hat pin) and carefully scrape the grout surrounding the
small hole to remove any flaking or loose grout.  Then, if the number
of holes and the total area is small, use your finger to firmly rub new
grout into the hole.  For larger areas you might use a small spatula.  
Even-out the new grout with the old, and wipe off the excess with a
paper towel.  Let it sit for a couple of hours.  You’re done!

Grout can be purchased at any hardware store, although a bag of
dry grout will be way more than you probably need.  Dry grout also
has limited shelf life because it absorbs moisture from the air as it sits
on the shelf.  If you do use dry grout, though, mix it into a very thick
paste; adding too much water to the mix results in a weak patch.

Premixed grout is available in small quantities which would probably
be sufficient for your needs.  Simply apply it directly from its container
without mixing.

Caulk: the eighth wonder of the world

Caulked joints are another potential problem area that need
observation and maintenance.  If any caulk beads have begun to
separate from their duty stations, they should be stripped out entirely
and new caulk applied.  Applying new caulk on top of old is an
absolute waste of time, in addition to being ugly.

Removing caulk can be a bit of a chore.  Using a razor knife (i.e.,
carpenter’s knife), slice each bead twice along its entire length, once
on each side.  Pull out the bulk of the bead, which should come out in
a single rubbery strip.  Then, using a sharpened spatula or a single-
edge razor, scrape the remaining residue off of each surface.  With a
little elbow grease surfaces can be made fairly clean, but they do not
necessarily have to be perfect.

Applying new caulk is most neatly accomplished by first masking the
two surfaces to be joined with masking tape.  On each of the
neighboring surfaces, place a length of masking tape along the entire
length of the joint to be caulked.  Locate the tape about 1/8” away
from the edge of each surface.  In the end, it will look like two parallel
pieces of tape with a sliver of space between them.

With a steady stroke of your caulk gun, lay an even bead of caulk in
the joint.  Use your finger to smooth over the wet caulk.  (The caulk
won’t hurt you, but latex gloves will make cleaning your finger
afterward easier.)  Immediately remove the masking tape.  Let the
whole thing sit for a couple/few hours.

See? It doesn’t have to be this way

That’s really all there is to it.  Simple maintenance doesn’t take that
much time.  Unfortunately, these menial tasks are typically
overlooked or ignored, and escalation to the next level is inevitable.