How about some kicking?

 A:

400 choice warm up.

4 x 150 w/:20 rest (50 choice kick/100 swim).

1 x 500 swim as (75 fast free, 25 easy non-free).

4 x 125 @ 2:15 (25 sprint kick/100 swim).

1 x 500 swim as (25 fast Tarzan drill/75 steady swim).

6 x [50 FAST @ :40, 25 easy @ :40]

200 cool down

*3200 total*

B:

300 choice warm up.

4 x 150 w/:20 rest (50 choice kick/100 swim).

1 x 400 swim as (75 fast free, 25 easy non-free).

4 x 125 @ 2:45 (25 sprint kick/100 swim).

1 x 400 swim as (25 fast Tarzan drill/75 steady swim).

4 x [50 FAST @ :50, 25 easy @ :50]

100 cool down

*2600 total*

C:

200 choice warm up.

2 x 150 w/:20 rest (50 choice kick/100 swim).

1 x 300 swim as (75 fast free, 25 easy non-free).

4 x 75 w/:15 rest (25 sprint kick/50 swim).

1 x 300 swim as (25 fast Tarzan drill/75 steady swim).

4 x [50 FAST @ :60, 25 easy @ :60]

100 cool down

*1800 total*