Trisuit for warm water event

There are some that will work well, but they will cost you maybe $200 or more.

I go with my tri top and shorts and a speed suit I bought off ebay for $20. Covers my pockets, seams and waist band, compresses a little to reduce more drag in the water. Easy to get off.

Partly this is a matter of the tri suit's fit. If you are comfortable with a tighter fit for the bike and run, then go a little smaller than you might for a wetsuit race. I have a 2XU suit that is fine for ocean swims without a wetsuit. It's XS; I have a similar suit that is size S that I wear for wetsuit races. Both feel fine on bike and run.

I swam competitively in HS and age group clubs and think I know drag when I feel it; the drag on a well-fitting tri suit is pretty minimal. If no open water is available, you can try pushing off aggressively from the wall in a pool--if you feel water under the suit or pockets flapping around, it is too loose to be ideal for a no-wetsuit swim. I definitely have some suits that I will only wear for wetsuit-legal races and some that I wear when I can't wear the wetsuit.

Otherwise, just invest in a cheap swim skin. Solves the problem of drag, makes you faster, allows you to wear a favorite suit even if it's not skin-tight, and is very easy to get on/off. It doesn't make you feel hotter in warm water, if that is a concern. I wore one (Roka Viper) for Vietnam 70.3 in last month, with air temps well into the 90's and water in the 80's, and it was fine.