I did a little extra research on this....
Enlarge Person lasts 1 min x your level = 3min
3min = 30 six second rounds
Effects of spell are…
This spell causes instant growth of a humanoid creature, doubling its
height and multiplying its weight by 8. This increase changes the
creature’s size category to the next larger one.
The target gains a +2 size bonus to Strength, a –2 size penalty to
Dexterity (to a minimum of 1), and a –1 penalty on attack rolls and AC
due to its increased size.
On Swimming...
SWIM (STR; ARMOR CHECK PENALTY)
Using this skill, a land-based creature can swim, dive, navigate
underwater obstacles, and so on.
Check: Make a Swim check once per round while you are in the water.
Success means you may swim at up to one-half your speed (as
a full-round action) or at one-quarter your speed (as a move action).
If you fail by 4 or less, you make no progress through the water. If
you fail by 5 or more, you go underwater. If you are underwater,
either because you failed a Swim check or because you are swimming
underwater intentionally, you must hold your breath. You can hold your
breath for a number of rounds equal to your Constitution score, but
only if you do nothing other than take move actions or free actions.
If you take a standard action or a full-round action (such as making
an attack), the remainder of the
duration for which you can hold your breath is reduced by 1 round.
(Effectively, a character in combat can hold his or her breath only
half as long as normal.) After that period of time, you must make a DC
10 Constitution check every round to continue holding your breath.
Each round, the DC for that check increases by 1. If you fail the
Constitution check, you begin to drown (see Suffocation and Drowning
in the Dungeon Master’s Guide).
The DC for the Swim check depends on the water, as given on the
table below.
Water Swim DC
Calm water 10
Rough water 15
Stormy water 20*
*You can’t take 10 on a Swim check in stormy water, even if you aren’t
otherwise being threatened or distracted.
Each hour that you swim, you must make a DC 20 Swim check or take 1d6
points of nonlethal damage from fatigue.
Action: A successful Swim check allows you to swim one-quarter of your
speed as a move action or one-half your speed as a full-round
action.
Special: Swim checks are subject to double the normal armor check
penalty and encumbrance penalty (see pages 123 and 162).
For instance, full plate incurs a –12 penalty on Swim checks instead
of –6.
KEVIN - In this case we'd be talking about a stormy water DC, and an
additional Constitution check for the sapping of strength that would
occur for every round your in the frigid water. DC would start at 10
and go up in increments of 5 every 2 rounds. For every failure 1d6
points of nonlethal damage from fatigue until you pass out and are
swept down river...of course then there would be the drowning... :(
Let me know if your still interested...I'd definitely award experience
points for this...course I might make you take a temporary deduction
in Wisdom. :)